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WORKS MANAGEMENT LIBRARY 



THE CORROSION 
OF IRON 

A SUMMARY OF CAUSES AND 
PREVENTIVE MEASURES 

BY 

L. C. VSTILSON 




NEW YORK 

THE ENGINEERING MAGAZINE CO. 

1915 






Cop\Tight, 1915 

By THE EXGIXEERING MAGAZINE CO. 



i 



y — 



DCT 301315 
©aA4iei47 



PREFACE 

This little volume is the result of an attempt to 
collect and put in simple form for reading and ready 
reference some of the more interesting and important 
facts connected with the corrosion of iron and its 
protection therefrom. 

It requires no argument to prove the widespread 
nature and serious import of this process of decay, 
and the volume of literature to be found on this sub- 
ject is evidence of the increasing attention which is 
being paid to it. It is only comparatively recently, 
however, that corrosion has been studied in a scien- 
tific manner and something of its true character as- 
certained, so it naturally follows that the same thing 
is true in large degree of the measures employed for 
protection. This is, perhaps, especially the case with 
paint pigments and vehicles, where careful experi- 
ments have brought to light many fundamental truths 
of the greatest value regarding the nature and action 
of some of these materials, thus enabling us to use 
them with much greater intelligence and effective- 
ness. Much of the published work, however, is scat- 
tered throughout many different scientific and tech- 
nical magazines, consequently is largely inaccessible 
to one who does not have the time or facilities for 
such investigation. 

It is accordingly hoped that this book, by pre- 
senting some of this material in condensed form, 
may help to give the student or busy engineer a 
better understanding of the problems involved in the 
successful preservation of one of our most useful 
building materials, and point the way to their solu- 
tion. 

Brooklyn, New York. 

July 31, 1915. 



CONTENTS 



Chapter I. The Rust Problem. 



Its Importance to Constructing Engi- 
neers, Architects and Owners — Susceptibil- 
ity of Iron and Steel to Corrosion — Eco- 
nomic Bearings of the Matter — What Cor- 
rosion Is — The Contest of Iron versus 
Steel — Effect of Modern High-speed Meth- 
ods of Steel-making — Remarkable Exam- 
ples of Preservation of Iron from Rust — 
Explanation of this Good Behavior of Cer- 
tain Articles — The Chemical and Physical 
Facts Involved in Corrosion — Theory and 
Process of Solution — Ions and Ionization — 
Theory of Dissociation — Precipitation — 
Practical Bearing of these Things on the 
Rusting and Protection of Iron and Steel. 

Chapter II. Theories op Corrosion ....... 23 

The Three Principal Theories— The Hy- 
drogen Peroxide Theory — Evidence for and 
against It — The Carbonic Acid Theory — 
Experiments Showing Its Partial Truth — 
The Electrolytic Theory — Data Supporting 
It — How It Agrees with Observed Facts — 
How It Influences the Choice of Protective 
Measures — Bad Influence of Stresses and 
Strains — Of Pickling — Of Indentations and 
Roughness of Surface — Conclusions. 

iii 



IV CON'TEN'TS 

Chaptee III. Protective Measuees 46 

Practical Application of the Principles 
Discovered — Three Methods of Protection: 
by Coatings of Oxide, by Coatings of Other 
Metals, by Covering with Paint or Similar 
Materials — Pioneer Work in Oxide Coatings 
by Lavoisier and Faraday — The Bower- 
Barff Process — The Wells Process — The 
Gesner Process — The Dewees-Wood Process 
— The Bontempi Process — The Buflfington 
Process — Heat Bluing — Coslettizing — Pro- 
tection by Coatings of Other Metals: Gal- 
vanizing by Dipping, Electrolysis, and 
Vapor — Hot Galvanizing — Methods of 
Testing — Electro-depositing of Zinc : Meth- 
ods and Defects — Tapor Processes — Sher- 
ardizing: Details and Advantages of the 
Process — The Molten Zinc Process — Preece 
and Walker Tests for Galvanized Work — 
Copper-coating — Nickel-coating — Lead- 
coating — Lohmannizing — The Schoop Proc- 
ess. 



Chapter IY. Paint Materials 74 

Wide Applicability of Painting as a Pro- 
tective Measure — Xatnre of Paint — Two 
Methods of Making Paint — Comparative 
Merits of these Different Procedures — In- 
gredients of Paint — Linseed Oil — Drying 
and Driers and Their Effects on the Qual- 
ity of the Oil — Adulterants of Linseed Oil 
— Other Paint Oils — Tung or China Wood 
Oil — Soya Bean Oil — Menhaden Oil — Thin- 
ners — Turpentine — Petroleum Oils — Pig- 



CONTENTS V 

ments — Their Influence on Durability — 
Findings of the Committees of the Amer- 
ican Society for Testing Materials Tabu- 
lated and Discussed — Pigments Described — 
Zinc Chromate — Zinc and Barium Chrom- 
ate — Zinc Oxide — Zinc-lead White — 
Barium Chromate — Ultramarine Blue — 
Chrome Green — Prussian Blue — Litho- 
pone — Willow Charcoal — Litharge — Basic 
Carbonate-White Lead — Calcium Sulphate 
— Princess Metallic Brown — Orange Min- 
eral — Calcium Carbonate — Sublimed Blue 
Lead — Lemon Chrome Yellow — ' Orange 
Chrome Yellow — Medium Chrome Yellow 
— Chrome Green — Venetian Eed — Bone 
Black — Asbestine — China Clay — Eed Lead 
— Indian Eed — American Vermilion — Sub- 
limed White Lead — Mineral Black — Barytes 
— Graphite — Ochre — Carbon Black — ■ 
Lampblack. 

Chapter V. Protective Paints 100 

Quality of Paint for Ironwork Depends 
on Inhibitive, Insulating and Excluding 
Properties — Action of Various Pigments 
Discussed — Tests of Paint Films — Cush- 
man and Gardner's Eesults — Tests Made 
by the American Society for Testing Ma- 
terials — Formulae of the Paints Tested — 
Eesults — Conditions Controlling the Char- 
acter of a Paint Film — How to Work up a 
Good Paint Formula — Effect of Various 
Ingredients — Japans and Enamels — Lac- 
quers — Preparatory Treatment of Iron and 
Steel Surfaces before Painting. 



VI CONTENTS 

Chapter VI. Influence of Different Ele^ 

MJENTS ON the CoRROSION OF IrON 127 

Definite Work of Eecent Date — Manufac- 
ture of Iron Summarized to Show Nature of 
the Problem — Natural Impurities and Their 
Effect on Electrolysis — Influence of Normal 
Constituents Individually Determined — 
Silicon — Sulphur — Manganese — Arsenic 
— Cobalt and Nickel — Silver, Tin, Tungsten 
and Selenium — Copper — Tests of Copper- 
Steels — Buck's Eesults Tabulated — Excel- 
lent Eesults Secured in Protection from 
Corrosion under Severe Conditions — Hope- 
ful Outlook for Copper-Bearing Steel as a 
Rust Eesister. 

Chapter VII. Corrosion of Wrought-Iron 

AND Steel Pipe 149 

Controversy over their Eelative Merits — 
Importance Due to the Enormous Dse of 
Pipe — Puddling and Bessemer Processes 
Summarized — Their Tendencies Pointed 
Out — ^Wr ought Iron Subject to Lamination 
— Bessemer Steel Free from this Tendency 
— Tests Made by the Navy Department — 
Eesults — Data Collected by the American 
Society for Testing Materials — Thomson's 
Experiments — Tests of the American So- 
ciety of Heating and Ventilating Engineers 
— Woolson's Tests in New York City Public 
Baths — Walker's Experiments on Pipe Lines 
— All Conclusions Negative to the Superior- 
ity of Iron over Steel as to Corrodibility, 
and Tendency to Pitting Somewhat Greater 
in the Case of Wrought Iron. 



THE CORROSION OF IRON 

Chaptek I 
THE EUST PEOBLEM 

A MONG the many questions which the ar- 
-^ ^ chitect and builder are called upon to 
consider, none perhaps, is more important 
than that relating to the corrosion of iron 
and steel. This tendency to decay is peculiar 
to these materials, since none of the ordinary 
metals exhibits it to any comparable degree, 
and it is so strong that an unprotected piece 
is soon reduced to a shapeless mass of rust. 
It is one thing to design and erect a mighty 
sky-scraper, but an entirely different matter 
to protect it from those influences which, if 
allowed to do their work, would ruin the 
greatest structure in a short time. 

It seems strange, in a way, that unless iron 
is well protected it is far less resistant to 
natural agencies than wood or other building 
materials. It is being used more and more 
in the fabrication of lar^e buildincrs and other 



2 COEEOSIOIT OF IRON" 

structures of a permanent type, and the 
length of their life, to say nothing of the 
safety of the people employed in and around 
them, depends on the success with which cor- 
rosion can be prevented. 

Painters are constantly employed on such 
structures as the Brooklyn or Forth bridges, 
scraping away rust spots and repainting. 
When serious rusting is once started it is 
very hard to check, and as it would inevitably 
lead to the destruction of the framework, a 
collapse would be certain to occur sooner or 
later, probably with an appalling loss of life. 

The economic side of the question is also 
of serious import. It is being recognized 
more and more that the increasing use of 
materials is depleting our stock of natural 
resources, and it is evident that when these 
are irrevocably lost through some form of 
decay, we have sustained a real and serious 
loss. The production of pig iron in the 
United States alone has increased amazingly 
until now many millions of tons are produced 
annually. How much of this is wasted for 
lack of suitable protection, or otherwise, can- 
not be stated very accurately, but the total is 
considerable. How long our ore supply will 
hold out against the steadily increasing de- 



THE RUST PROBLEM 3 

mand can only be vaguely guessed, but if 
present conditions keep up there is certain to 
come a time when the scarcity and high price 
of iron will not permit it to be used so prod- 
igally as now. Again, the coal supply is by 
no means unlimited, and since it requires 
about four tons of coal, or its equivalent, to 
produce one ton of steel, it is apparent that 
when we allow iron to rust or otherwise go 
to waste we are losing not only iron, but coal, 
one of our most valuable assets. 

I do not mean to imply that the corrosion 
of iron is a new problem, or that technolo- 
gists generally do not realize its importance, 
since the articles and discussions which have 
been appearing in the technical press for the 
last few years are evidence of the work that 
is being done along these lines. This litera- 
ture, however, is distributed through a large 
number of publications and is, therefore, not 
readily accessible to the busy engineer, so 
in this small volume an attempt will be 
made to collect some of this information re- 
garding the study of corrosion and its prac- 
tical application in the protection of iron 
and steel, and to present it, together with the 
results of personal observations and tests, in 
a simple, compact form. 



4 COEEOSION" OF lEON 

Before taking up any of the theories as to 
the nature and cause of corrosion, it may be 
interesting to note a few points about the 
process in general. It is a matter of common 
knowledge that iron soon becomes covered 
with a heavy coating of rust, if it is exposed 
to moisture without first being protected by 
paint or other good preservative, and that in 
time the piece will be entirely honeycombed 
and will assume a lace-like appearance. It 
is also known that different pieces of iron and 
steel exhibit widely varying tendencies in this 
respect ; that is, some are surprisingly resist- 
ant and keep in fairly good shape for a long 
time even under very adverse conditions, 
while others soon show the most serious rust- 
ing when exposed to the same conditions. 
The manufacturers of wrought and charcoal 
irons and the various kinds of steel have each 
contended strenuously that his particular 
product is most resistant to corrosion, but it 
is not the province of this chapter to state 
who is in the right, since it appears that each 
of these types of metal may vary greatly in 
rust resistance and that there are good and 
bad irons as well as good and bad steels. 

There is plenty of good evidence to support 
this view, and it seems worse than useless, 



THE RUST PROBLEM 5 

therefore, for mill men to engage in wordy 
controversy over tlie particular merits of 
their products. The recent researches of sev- 
eral investigators show that internal stresses 
or strains in a metal, as well as improper 
treatment resulting in occluded gas and a 
porous metal with blow holes, are conducive 
to rapid corrosion. Modern high-speed meth- 
ods, unless carefully performed, tend to pro- 
duce a metal in which various substances, 
which are present either as impurities or are 
added to produce certain effects, are more 
or less segregated instead of being thor- 
oughly mixed, and the iron is pounded and 
rolled into shape and cooled so quickly that 
the molecules do not have time to adjust 
themselves and heavy internal strains may 
be produced. Of course, the remedy for all 
this is more careful workmanship and ade- 
quate annealing. Metal which is strained 
beyond its elastic limit can be shown to be 
more easily corroded than metal which has 
not been strained, and it apparently makes 
little difference whether these strains are 
produced during the process of making the 
part or afterward. 

There seems to be no doubt that at least 
some of the metal produced previous to the 



b COEROSIOX OF IROX 

introduction of modern metliods (modem as 
far as speed is concerned) was superior in 
its resistance to corrosion to tlie present-day 
product, although the latter should not be 
condemned too hastily on this one point alone. 
Thus, I have seen various iron articles, espe- 
cially nails, which showed far less rust after 
an exposure of a hundred years or so, than 
the modern variety sometimes does in a few 
weeks. 

Another piece, an old flint-lock pistol, was 
especially interesting. It was found by a 
friend in a patch of woods in Vermont and 
had evidently lain there for many years, since 
a piece of newspaper dated 1796 had been 
used as wadding in loading it. All the iron 
parts were rather rough and pitted and cov- 
ered with rust, but the arm was in surpris- 
ingly good shape, considering the conditions 
to which it had been subjected. The spring, 
hammer, and trigger were still capable of 
performing their functions and very little 
effort was required to put the old weapon in 
decidedly presentable condition after its ex- 
traordinary exposure to the weather. 

The old bridge at Newburyport, Mass., has 
also attracted much attention because after 
something like a hundred years' exposure to 



THE RUST PROBLEM 7 

the elements it shows remarkably little evi- 
dence of deterioration. 

Going farther back, a number of iron arti- 
cles have come down to us from ancient times, 
and perhaps the most noted of these is the 
famous pillar in the temple of Kutab Minar 
at Delhi, India. This old shaft, which pro- 
jects some 30 feet above the surface of the 
ground, was erected about 900 B. C. Today 
it shows little trace of rust, although it has 
had no protective coating other than that 
which the process of manufacture and the 
atmosphere itself have formed upon it. 

In this connection mention may also be 
made of what are known as the Ceylon link 
and the Singhalese chisel, nail and bill hook, 
all specimens of great antiquity. 

To explain how or why these various arti- 
cles have so successfully withstood the at- 
tacks of air and moisture for so many years 
is not as easy as it might seem, for appar- 
ently some of our commonly accepted notions 
and theories as to what a good iron should 
and should not be do not hold, or at least are 
offset by other conditions. For instance, 
some of the links of the Newburyport bridge 
have been examined and found to be of very 
ordinary purity and of a most heterogeneous 



8 CORKOSIOIT OF IRON 

structure, segregation of the impurities being 
decidedly noticeable. The question of any 
inherent superiority in the iron itself was 
settled by heating and rolling one of the links 
into a sheet and exposing it to the weather, 
when it was found that it corroded quite as 
readily and extensively as the material of to- 
day. 

Again, tests made on the old relics referred 
to above show that only in manganese content 
do they approach modern irons in composi- 
tion; the phosphorus is from 22 to 60 times 
and the silicon from 9 to 50 times as high in 
these as in the present product. From this 
it would seem that on the score of composi- 
tion they are in no way superior to our own 
material, and in addition they contain several 
per cent of slag and cinder, which are admit- 
tedly very undesirable. 

The explanation of their good behavior in 
service is probably found in the fact that the 
iron from which they were produced was re- 
fined, and the objects themselves fashioned 
slowly and laboriously by hand in a sort of 
forge. The prolonged heating and slow forg- 
ing would tend to insure thorough annealing 
and freedom from strains, while the constant 
hammering produced a dense skin on the sur- 



THE BUST PROBLEM 9 

face, and this was doubtless covered with a 
more or less heavy coating of scale or oxide 
which also helped to protect the metal be- 
neath. Thus it seems that the ability of these 
irons to withstand atmospheric attack is in- 
cidental to the method of working, rather 
than due to greater purity or freedom from 
segregation, as compared with modern irons. 
Of course, to apply such methods to the enor- 
mous output of iron and steel nowadays 
would be utterly impossible. 

Mention has been made of the impurities in 
iron, and it may not be out of place to enum- 
erate some of them at this time. All except 
very highly purified iron contains silicon, sul- 
phur, phosphorus, manganese, and combined 
carbon, at least in small amounts. In making 
alloys, chromium, vanadium, tungsten, nickel, 
copper and several other elements may be 
added. Iron alloys or combines with prac- 
tically all the other elements, and is so easily 
influenced by some of them that oftentimes 
only a very small percentage is required to 
alter its character markedly; as a conse- 
quence, specification requirements are gener- 
ally drawn very rigidly. 

A good deal of importance has been at- 
tached to the influence which sulphur, phos- 



10 . COREOSION" OF IROK 

phorus, and the other normal constituents 
of iron are supposed to exert on the process 
of corrosion, and manganese, in particular, 
has come in for an unusual amount of con- 
demnation. For example, it is known that it 
decreases the electrical conductivity of iron, 
to a certain point ; therefore, unless it is uni- 
formly distributed, the electrical conductivity 
will vary in different parts and on different 
surfaces, which has an important bearing on 
the problem in hand, as will be seen later on. 
Further, it is well known that manganese 
and sulphur tend to combine when both are 
present in iron and steel, and the resulting 
sulphide shows a difference of electrical po- 
tential against iron, which is a highly unde- 
sirable condition. In general, it may be said 
that, according to the electrolytic theory, any 
condition which tends to produce a difference 
of potential between surrounding parts is 
bad and will excite or accelerate rapid corro- 
sion. 

A detailed discussion of the influence of 
the various elements, as far as corrosion is 
concerned, will be taken up later on, but it 
may be said here that the results of a vast 
amount of experimental work show that even 
extremely pure iron corrodes about as readily 



THE RUST PROBLEM 11 

as a good, commercial grade of material; 
therefore the question is apparently not so 
much what amount of impurities may be 
present as whether these are thoroughly dis- 
tributed throughout the metal or segregated 
here and there in patches. 

Before we can discuss some of the theories 
regarding the formation of rust and thus 
approach the subject in a scientific manner, 
however, it will be necessary to review briefly 
certain chemical and physical facts involved 
in the process, in order that there may be no 
misunderstanding of the terms and principles 
employed. Primarily we will be concerned 
with the phenomenon of solution. 

Water is the great solvent for all forms of 
matter and there is probably nothing which 
it does not affect in some way. Most chemical 
changes take place in water, or in its pres- 
ence, so it is not only destructive in its action, 
as we often feel, but also constructive, as 
new compounds may be formed as well as old 
ones broken up. When we speak of the solu- 
bility of a substance we refer to the ease 
with which it goes into solution; therefore 
we say it is insoluble if we find that no appre- 
ciable amount has been dissolved. Prac- 
tically, however, there is almost nothing 



12 CORROSION" OF IRON" 

which is not dissolved to some extent, so the 
term is only relative, and there are all de- 
grees of solubility ranging from those cases 
in which the solvent dissolves but an in- 
finitesimal amount of the material, to those 
in which it takes more than its own weight of 
the substance into solution. 

This is not the place to discuss why a cer- 
tain solid does or does not dissolve in a cer- 
tain liquid, so we will confine ourselves to 
substances which are easily recognized as 
soluble in water, that is, dissolve readily in it. 
When such a body is placed in water it dis- 
appears from view more or less rapidly, and 
its molecules, if it is a compound body, dis- 
tribute themselves uniformly among the 
molecules of the solvent. This distribution is 
produced by a very definite force, known as 
the ** solution pressure,'' which is exactly an- 
alogous to the pressure exerted by a gas con- 
fined in a closed vessel. Under such circum- 
stances the gas tends to occupy all the space 
at its disposal, and therefore produces a cer- 
tain uniform pressure on all parts of the 
vessel. If we had steam at a pressure of 
100 pounds per square inch in a boiler and 
wished to introduce more steam into the lat- 
ter, it is evident that we would have to have 



THE RUST PROBLEM 13 

the incoming vapor at a pressure slightly 
higher than 100 pounds per square inch, in 
order to overcome the back pressure, as it is 
called. Again, as a boiler is generating 
steam and the tension increases, it becomes 
ever harder for the water to assume the vapor 
state, against the pressure of the steam al- 
ready formed. 

The same thing happens in the case of so- 
lutions. As more and more of the substance 
dissolves, there is produced a back pressure, 
known as osmotic pressure. It is evident that 
when the osmotic and solution pressures are 
equal, no more of the material will go into so- 
lution and a state of equilibrium will result. 
If any outside influences disturb this equilib- 
rium and lower the osmotic pressure, for 
example, it is easily seen that more of the 
substance will pass into solution until equilib- 
rium is again established. Osmotic pres- 
sure may be of surprising magnitude, in cer- 
tain cases, and can be shown by placing a 
glass tube closed at one end by animal parch- 
ment and filled with alcohol, in a glass of 
water. Alcohol and water have a strong ten- 
dency to diffuse or mix, and water can easily 
pass through the parchment into the tube, 
but the alcohol cannot; therefore the column 



14 CORROSION" OF IRON" 

of liquid in the tube will rise, showing an 
increased pressure. It may be interesting to 
note that the solution pressure of a very 
soluble substance, like sugar, is considerable. 
One investigator found that the pressure in 
the case of a 6 per cent sugar solution 
amounted to nearly 60 pounds per square 
inch. 

Coming now to the behavior of substances 
in solution, we know that if we dissolve salt 
in water it apparently disappears, but since 
we realize it must be present in some form 
we conclude that it has been resolved into ex- 
tremely fine particles. Experiment has shown 
that such a solution will conduct a current of 
electricity and two entirely new substances 
will make their appearance, one around each 
electrode, so it is plain that something in the 
solution is capable of carrying the electricity 
from one pole to the other, and that in so 
doing certain changes have taken place. The 
term *4on" was introduced, therefore, to 
denote the particle of matter which travels 
in the solution, along with the electricity, 
toward the poles. A further distinction is 
made in that the particle of matter which 
travels toward the anode or positive pole is 
called an an-ion, and that which migrates 



THE RUST PROBLEM 15 

toward the cathode or negative pole, a cat- 
ion. 

The theory of dissociation holds, at the 
present time, that when acids, bases, and 
salts are dissolved in water they break down 
into ions, which are considered to be atoms 
or groups of atoms carrying large charges 
of static electricity. These charges are not 
apparent in the undissociated molecule, be- 
cause the positive charges exactly neutralize 
the negative ones. Thus, hydrochloric acid 
dissociates into positive hydrogen and nega- 
tive chlorine ions; sodium hydroxide (caustic 
soda) forms positive sodium and negative 
hydroxyl ions; ferrous chloride forms posi- 
tive ferrous and negative chlorine ions, and 
soon. 

Putting it in the form of a principle, with- 
out taking the time to demonstrate it, we may 
say that the cation of all acids is hydrogen; 
the anion varies with the acid. Likewise, the 
anion of all bases is the hydroxyl group 
(OH) ; the cation varies with the base, the 
metal with which it is combined. Since hy- 
drogen is the characteristic ion of all acids, 
it can be seen that whenever its ions are pres- 
ent, acid properties are exhibited; in like 
manner, hydroxyl is the characteristic ion of 



16 COREOSION" OF lEON" 

all bases, and wlien liydroxyl ions are present 
basic properties are in evidence. With salts, 
both the anion and cation vary according to 
the elements composing the salt. It is not 
possible to discuss this interesting topic fur- 
ther, but anyone who wishes more informa- 
tion along these lines should consult some 
good text book on physical chemistry. Per- 
haps the application of the foregoing to the 
corrosion of iron may seem a little obscure, 
so we shall anticipate enough to say that this 
process is greatly retarded or accelerated by 
the presence of different ions, therefore some 
knowledge of their formation and action is 
necessary. 

Another form of breaking down or disso- 
ciation, known as hydrolysis, is also of im- 
portance in the study of corrosion. If a solu- 
tion of the carbonate, say, of sodium, potas- 
sium, or other strongly basic metal be tested 
with litmus paper, it will be found to have 
a strongly alkaline reaction. On the other 
hand, chromium, bismuth, aluminum and 
other weak bases tend to form salts which 
have an acid reaction; some are even too 
weak to exist. The principle may be deduced, 
therefore, that a compound formed from a 
weak acid and a strong base will be alkaline 



THE RUST PROBLEM 17 

in reaction, while one formed from a strong 
acid and a weak base will be acid. 

Wben considering the theory of dissocia- 
tion it was said that hydrogen ions are the 
cause of an acid reaction and hydroxyl ions 
the cause of an alkaline reaction. Conversely, 
it can be said that an acid reaction denotes 
the presence of hydrogen ions, while hy- 
droxyl ions are shown by an alkaline reac- 
tion. It has just been noted that different 
ions may exert great influence on the process 
of corrosion ; therefore, when we go into this 
more deeply later on we should be able to ex- 
plain, in many cases at least, why certain pig- 
ments are stimulators of corrosion, or why 
they tend to act on the oil in paint films and 
cause early decay and failure. Basic car- 
bonate of lead — white lead — for example, is 
so alkaline in its reaction that it saponifies 
or makes lead soaps of the linseed oil, leading 
to what is called chalking and deterioration 
of the paint film. 

A further investigation of solutions shows 
that some will conduct a current of electricity 
while others will not. A substance which will 
conduct a current when in solution is called 
an electrolyte. For example, potassium ni- 
trate, saltpetre, in solution will conduct a 



18 CORROSION- OF IRON 

current very readily, but syrup offers a high 
resistance to its passage. It can be shown 
that potassium nitrate ionizes highly in so- 
lution, which is not true of sugar, and it has 
been noted previously that when a current is 
passed through a conducting solution a me- 
chanical movement of the ions takes place 
toward the electrodes, where they give up 
their charges and ^ Opiate out'' in metallic or 
gaseous form. This may be familiar to the 
reader as the process of electro-plating. If 
a solution of zinc sulphate is made to conduct 
a current, the positive zinc ions proceed to 
the negative pole, give up their charges, and 
assume the atomic condition. An equivalent 
amount of sulphuric acid is set free at the 
same time at the positive pole. If this is of 
zinc, as in electro-galvanizing, the acid at- 
tacks it and a further quantity of metal is 
taken into solution. Going a step farther, it is 
not necessary that an external source of elec- 
tricity be provided before electrolysis can 
take place. Strips of two different metals 
will generate a current when they are put into 
an electrolyte and the upper ends touched to- 
gether or joined by wire. This may be ex- 
plained by saying that the more electro-posi- 
tive metal shoots positive ions into the solu- 



THE EUST PROBLEM 19 

tion and thus leaves itself negatively charged, 
consequently appearing as the negative pole. 
It is more or less rapidly disintegrated, while 
the other is protected. As will be shown later 
on, this phenomenon plays a very important 
part in the preservation of iron, especially 
by zinc coatings. 

As everyone knows, all but a few of the 
elements are capable of combining and form- 
ing compounds with the other elements. The 
capacity of an atom to combine with other 
atoms is known as its '^valence'' and varies 
with the different elements. For instance, 
one atom of chlorine can combine with only 
one atom of another element; one atom of 
zinc can combine with two monovalent atoms, 
and so on. Sometimes the same element va- 
ries in its ability to combine with others ; for 
example, copper combines sometimes with 
one and sometimes with two atoms of chlor- 
ine. Manganese, gold, iron, and many others 
exhibit this peculiarity, which is thought to 
be due to the electrical state of the ion. By 
appropriate means such an element may be 
made to combine with a larger quantity of 
another element, if it is in the lower state of 
oxidation, or to give up a part of the element 
with which it is combined, if it is in the higher 



20 SOEROSiON OF IRON" 

state. Passing from a higher to a lower state 
of valence is known as '* reduction" and from 
a lower to a higher state as ** oxidation." 
Often one of these states is very unstable, 
tending to pass into the other under ordinary- 
circumstances. Thus iron forms two classes 
of compounds, ferrous and ferric, which have 
a valence of two and three, respectively, but 
the ferrous compounds oxidize very easily to 
the ferric state. 

If we immerse pure iron in pure water a 
certain amount of it will be dissolved, since 
iron has a small but definite solution pressure 
in water. It goes into solution as ferrous 
hydroxide; therefore, the iron is present as 
positive ferrous ions, the hydroxyls taking 
the negative charge. As soon as the ferrous 
ions come into contact with the oxygen of the 
air they are oxidized to the ferric state. Fer- 
ric hydroxide is insoluble in water, therefore 
it precipitates out ; and since its influence on 
the solution is then at an end, more iron will 
dissolve and so the process will be repeated 
over and over until all the iron is used up. 
Meanwhile the precipitated iron compounds 
accumulate and the rust rapidly grows 
thicker. 

We referred a moment ago to the process 



THE RUST PROBLEM * 21 

of precipitation and it will be interesting to 
consider briefly some of the properties of the 
product of this action. Under certain condi- 
tions, precipitates may take the form of very 
fine crystals and are termed ** crystalloids, " 
or they may appear in a gummy, gelat- 
inous condition and are then called ** col- 
loids," a word which is derived from the 
Greek word meaning glue. There are two 
important properties of colloids which should 
be carefully noted. The first is their peculiar 
propensity to absorb large amounts of liquids 
and impurities from the solutions from which 
they were precipitated. Since many pig- 
ments are first formed as colloidal precipi- 
tates, it will be seen that a serious source of 
danger is presented as the absorbed impuri- 
ties may more than neutralize any inhibitive 
qualities of the pigment itself. The second 
point of interest is the fact that colloidal 
precipitates suspended in an electrolyte be- 
have just as ions do, carrying electrical 
charges and migrating to the electrodes when 
a current is passed. Colloidal ferric hydrox- 
ide and all basic hydroxides collect around 
the cathode, therefore they may be consid- 
ered to have a positive charge. Hydroxides 
of an acid nature move toward the anode 



22 CORROSION OF IRON 

and thus have a negative charge. The most 
likely explanation of this is that a basic hy- 
droxide, for example, would split off negative 
hydroxyl ions and leave the residue with a 
positive charge, while an acid hydroxide 
would split off positive hydrogen ions, leav- 
ing the remainder with a negative charge. 

The practical significance of these facts is 
that they help to explain why the surface 
of a piece of badly rusted iron shows, as 
everyone has observed, many small but dis- 
tinct pits or depressions at some places and 
wart-like growths at others. Under the influ- 
ence of the electric currents formed during 
the process of corrosion, the ferric hydroxide 
is transported from the points having a posi- 
tive charge, leaving tiny depressions, and de- 
posited at points of negative polarity, thus 
raising these above the rest of the surface. 
This feature will be taken up later. 



Chapter II 
THEORIES OF COREOSION" 

TN the preceding chapter we considered 
■■■ some of the fundamental principles of 
physics and chemistry on which the explana- 
tion of corrosion is based so that now we are 
in better condition to undertake the study of 
this subject itself. Speculation concerning 
this process has been rife for a great many 
years and various theories have been put 
forth from time to time which have endeav- 
ored to explain satisfactorily all the observed 
facts and, incidentally, point the way to a 
remedy. Some of these theories could not 
stand rigid investigation and consequently 
fell by the roadside, but three more or less 
distinct propositions have gradually been 
evolved, and practically all students of this 
subject are adherents of some one of them. 
The hydrogen-peroxide theory was based 
on a scheme of oxidation proposed by a Ger- 
man chemist, Traube. It assumes that when 
iron, water, and oxygen are in contact, a re- 

23 



24 



COEEOSION" OF IRON" 



action takes place in which ferrous oxide (ox- 
ide of iron) and hydrogen peroxide are 
formed. The ferrous oxide then reacts with 
one-half of the hydrogen peroxide to form a' 
basic or hydrated oxide; rust, an insoluble 
basic oxide, is the final product. Part of the 
excess of peroxide then acts on more iron, 
forming ferrous oxide and water, whereupon 
more of the peroxide reacts with the iron 
compound to produce a further quantity of 
basic oxide, and so the process is repeated in- 
definitely. Perhaps these relations may be 
made clearer by the following chemical equa- 
tions : 



Fe 
Iron 



+ 



O2 
Oxygen 



+ 



H2O 

Water 



2Fe0 

Ferrous 
oxide 
Then, 

Fe 
Iron 

2FeO 

Ferrous 

oxide 



+ 



+ 



+ 



H2O2 
Hydrogen 
peroxide 

H2O2 

Hydrogen 

peroxide 

H2O2 

Hydrogen 
peroxide 



FeO 

Ferrous 
oxide 
Fe202(OH)2 = 
Hydrated 
oxide 



FeO 

Ferrous 

oxide 

Fe202(OH)2 

Hydrated 

oxide 



+ 



+ H2O2 

Hydrogen 
peroxide 
Fe203,H20 
Rust 



H2O 

Water 

Fe203,H20 

Rust 



As a whole, this theory has seemed to de- 
rive some support from the fact that slight 
traces of hydrogen peroxide have been de- 
tected during the slow oxidation of certain 
metals. However, the same tests have not 
shown this substance to be present while iron 
is rusting. 



THEORIES OF CORROSION" 35 

Another argument which, for a time, 
seemed to be strong evidence for the theory- 
was the observation that certain oxidizing 
agents, as chromic acid and its salts, prevent 
rusting when iron is immersed in water con- 
taining them; and hydrogen peroxide is 
known to be broken up by chromic acid. It 
has been pointed out, however, that because 
certain substances destroy hydrogen perox- 
ide, they do not necessarily prevent its for- 
mation, a contention which is borne out by 
analogous reactions with other compounds. 
A strong argument against the theory is the 
fact that other oxidizing agents which de- 
stroy the peroxide do not act the same as 
chromic acid and prevent rusting. Alto- 
gether, this theory does not seem to be suffi- 
ciently in accord with the known facts to 
serve as a working thesis. 

Of the two remaining theories, the carbonic 
acid and the electrolytic, the former has had 
the greater number of adherents and until 
recently has been considered to explain satis- 
factorily the phenomena of corrosion. As a 
matter of fact, it does express the truth to a 
certain degree, as will be shown later. It 
postulates that rusting is always started by 
an acid, even one as weak as carbonic suffic- 



26 CORROSION OP lEOlT 

ing to do this, the iron being first converted 
into a ferrous salt, with the escape of free 
hydrogen. Under the influence of oxygen 
and water the ferrous salt is oxidized to fer- 
ric hydroxide (which is insoluble and settles 
out) at the same time that the carbonic acid 
which was required to form the ferrous salt 
is set free. This again attacks the metallic 
iron with the formation of more ferrous com- 
pound which is again decomposed, ferric hy- 
droxide settling out and carbonic acid being 
set free to attack more iron and thus make 
the process a continuous one. The action may 
be shown by these equations : 

2Pe + 2H2CO3 = 2FeC03 + 2H2 

Iron Carbonic Ferrous Hydrogen 

acid carbonate 

2FeC03 + 5H2O + O = 2Fe(OH)3 + 2H2COS 

Ferroua Water Oxygen Ferric Carbonic 

carbonate hydroxide acid 

In order to prevent rusting, therefore, all 
we have to do is to neutralize the acid with 
an alkali, and those who held this to be the 
true explanation of the cause of corrosion 
pointed to the fact that iron does not rust in 
strongly alkaline solutions. It can be shown, 
however, that iron rusts very easily in dilute 
alkaline solutions, so that bit of evidence is 
thereby weakened. 



THEORIES OF COEROSION 27 

Various investigators have lined up on 
both sides of the question and have defended 
their views with a series of ingenious and 
painstaking experiments, the object of which 
was to determine whether or not iron can 
rust in oxygen and water in the complete ab- 
sence of carbonic acid. The result of this 
work seems to show that iron will corrode 
rapidly in the absence of this substance if 
oxygen and liquid water are present, and 
the experiments were so carefully made that 
no doubt remains that carbonic acid is only a 
slight factor in the corrosion of iron and that 
this process can go on very readily without 
it. In this connection it may be interesting to 
cite an experiment which Cushman and Gard- 
ner describe in their book on this subject. 
Two Jena glass flasks were nearly filled with 
freshly distilled water and boiled vigorously 
one-half hour, at the end of which time 
bright, polished strips of charcoal iron and 
steel were slipped into the flasks and the 
boiling continued a few minutes longer. The 
flasks were then completely filled with the 
boiled water and every trace of air removed. 
No rust appeared on the strips, even when 
they were kept indefinitely in this boiled, 
water. Pure oxygen was then freed from 



28 COEROSION" OF IRON" 

every possible trace of carbonic acid and 
passed into the flasks, whereupon rust formed 
on the bright pieces in about five minutes and 
was heavy in an hour. The action seemed 
to take place in patches having the appear- 
ance of a pattern, corresponding to the physi- 
cal structure of the metal. This experiment 
was also made using phenolphthalein, a sub- 
stance which shows the presence of exceed- 
ingly small amounts of acid or alkali, and in- 
variably a pink color developed, showing that 
no carbonic acid was present. This would 
seem to be good evidence, therefore, that 
water and oxygen can cause corrosion with- 
out the aid of other substances. 

The experiment was also tried of substi- 
tuting pure carbonic-acid gas, perfectly free 
from oxygen, in place of the pure oxygen 
used in the above tests. Under these circum- 
stances, no apparent action took place, even 
after several hours, although no doubt a 
slight amount of iron went into solution as 
the carbonate. When pure oxygen was al- 
lowed to enter and mingle with the carbonic- 
acid gas rusting began very shortly, a char- 
acteristic blue-green color, which always ac- 
companies the early formation of rust in the 
presence of carbonic acid, being noticed. In 



THEORIES OF CORROSION" 29 

normal rusting in the air this color is not 
to be seen. 

It can be further shown that, even if it is 
not possible to remove all traces of carbonic 
acid from the apparatus used in these experi- 
ments, the hydrogen ions which would be 
given by the dissociation of small amounts of 
this substance would not be much greater in 
number than those furnished by the dissocia- 
tion of pure water. On the whole, it may 
be said that the carbonic-acid theory ex- 
presses a partial truth, in that hydrogen ions 
must be present before iron can be attacked — 
a point which will be taken up later. 

The electrolytic theory is steadily gaining 
adherents because it seems to offer a satis- 
factory explanation for a greater number of 
observed facts than do any of the others. 
The modern conception of the reactions which 
take place between different substances in 
the presence of water holds that they are ac- 
companied by certain readjustments of the 
electrical state of the reacting ions, and ac- 
cording to the electrolytic theory iron must 
go into solution as a ferrous ion before it can 
be oxidized in the wet way. 

When metallic iron is put into a solution 
of copper sulphate (blue vitriol) the copper 



30 CORROSION" OF IRON" 

is precipitated in metallic form and the iron 
goes into solution, the ions of this taking the 
electrical charge of the copper ions. Now 
hydrogen is a metal, or at least it behaves 
like one, and has a lower solution tension 
than, iron; therefore if we place a strip of 
iron in a solution containing hydrogen ions, 
its atoms will assume an electric charge and 
it will be dissolved, as it was in the copper 
solution, while hydrogen will give up its 
charge, changing from the ionic to the atomic 
condition and escaping as a gas. Ferrous 
salts, especially in solution or freshly precipi- 
tated, are rapidly oxidized to the ferric state 
by atmospheric oxygen, so that all the elec- 
trolytic theory must do is account for the 
solution of the iron in the first place. 

It has been shown by the careful experi- 
ments cited that iron is slightly soluble in 
pure water, but as long as no oxygen is per- 
mitted to act on the solution no sign of cor- 
rosion can be seen. When it is admitted, 
however, the red ferric hydroxide soon pre- 
cipitates out. So, from the results of these 
and other experiments, there can hardly be 
any doubt that iron dissolves in pure water 
without the aid of other substances. 

A little reflection will show that before 



THEORIES OF CORROSIOlT 31 

iron can rust it must first go into solution, 
at the same time that hydrogen is set free, 
and oxygen must be present to oxidize the 
ferrous salts formed. An exchange of elec- 
tricity between the reacting ions is therefore 
demanded and the case may be considered 
to be one of electrolysis, as the disappearance 
of a hydrogen ion, as the hydrogen goes off 
as gas, means the appearance of a ferrous 
ion somewhere else. Going a step further, it 
can be seen that corrosion is due to the iron 
being attacked by hydrogen ions and taken 
into solution, so we should naturally suppose 
that anything which increases the concentra- 
tion of these ions would increase the rapidity 
of corrosion and, conversely, anything which 
decreases their concentration would tend to 
retard or prevent it. And this is precisely 
the case. It was shown in the preceding chap- 
ter that hydrogen is the characteristic ion 
of acids and hydroxyl that of bases; there- 
fore we should expect acids to stimulate cor- 
rosion and alkaline substances to inhibit it, 
which is in accord with fact. 

Taking a bird's-eye view of the process of 
corrosion as explained by the electrolytic 
theory, we see that it lines up about as fol- 
lows. When iron and water are brought to- 



32 COEEOSIOX OF lEOX 

gether a certain amount of the metal goes 
into solution, since it has a certain solution 
pressure — or, to put it a little differently, 
water even when very pure is dissociated 
slightly into hydrogen and hydroxyl ions, the 
former attacking the iron as outlined before. 
It can be shown that the solution tension va- 
ries at different parts of the surface, this 
variation being due to strains and segrega- 
tion of impurities; impurities in the solvent 
also tend to increase the solution tension. 
Therefore, the points where this is greatest 
will be positive to those at which it is of les- 
ser degree, and a current will flow between 
them if there is a conducting medium present. 
Eemembering that hydrogen acts like a 
metal, it is seen that its ions will tend to col- 
lect around the negative poles, while the 
hydroxyl ions will move toward the positive 
poles and try to exchange their static charges 
with the iron, so that the latter will go into 
solution and the hydrogen escape as a gas. 
If the hydrogen ions are in sufficient concen- 
tration, that is, if an acid is present, this ex- 
change of charges takes place very rapidly 
and the iron is quickly dissolved. Under or- 
dinary conditions, however, the action pro- 
ceeds much more slowly as the concentration 



THEORIES OF CORROSION" 33 

of the ions is comparatively low. Iron goes 
into solution, nevertheless, as can be shown 
by means to be described shortly, and for 
every ferrous ion which is formed a hydroxyl 
ion must be formed also and appear else- 
where. The oxygen of the air now acts on 
the ferrous salts and changes them into the 
ferric state, in which form they precipitate 
out as the familiar red rust, and so the proc- 
ess is repeated indefinitely. 

Mention has already been made of so-called 
indicators, substances which show by a 
change of color the presence of very small 
quantities of acids or alkalis, and these are 
employed to show the positive and negative 
poles. Through the work of several investi- 
gators the reagent known as f erroxyl was de- 
veloped. It consists essentially of phe- 
nolphthalein, which shows the negative poles, 
and potassium ferricyanide, which indicates 
the positive poles, in a solution of gelatin or 
agar. It has proved to be a very valuable 
help in these lines of study and has widened 
our knowledge considerably. 

Nearly everyone has observed that not all 
pieces of iron rust in the same way. Some 
are covered more or less evenly with a coat- 
ing of rust which is of fairly uniform thick- 



34: COEEOSIOX OF IROX 

ness ; others are deeply pitted in some places, 
while adjacent parts of the surface may show 
far less corrosion than might be expected. 
Without the aid of the f erroxyl test it is diffi- 
cult to explain just why these things should 
be. 

When a strip of iron is properly mounted 
in this reagent, various parts of the surface 
develop either a red or a blue color after a 
few hours, thus showing where the positive 
and negative poles are, which is equivalent 
to saying where the iron is being attacked by 
hydrogen ions and where it is being protected 
by the presence of the hydroxyl ions. Often 
these poles reverse and a positive pole be- 
comes negative, or vice versa; in fact, this 
reversal may be repeated several times. In 
this connection, perhaps it would be best to 
describe the results of such a test made by 
Cushman and Gardner and given in their 
book, to which reference has already been 
made. A strip of steel was placed in f erroxyl 
and carefully observed. 

"When the colors first developed two dark blue 
nodes formed at the opposite ends of the test piece, 
with a large pink area at the centre, where for a time 
the metal remained quite bright. Yery slowly, how- 
ever, the poles changed and the pink central area 



THEORIES OP COREOSION 35 

disappeared and gave way to a large blue node which 
enveloped three-quarters of the test piecO;, with a 
small opposed pinkish spot. Again and again a re- 
versal and change of poles took place and at least 
five such changes could be observed. As a result of 
this action the metal strip was rapidly covered over 
its entire surface with the same superficial, loosely 
adherent coating of hydroxide which is obtained in 
many cases when certain samples of iron and steel 
are allowed to rust under a layer of water. It is pre- 
sumable that as the surface of the metal is eaten into 
by the solution of the iron at the positive poles, a 
new condition of equilibrium occurs, resulting in 
changes and even reversal of the positive and negative 
nodes. This would indicate that in the case of metals 
which suffer from local action or pitting, the segre- 
gation conditions are of a different nature from those 
which exist in the case of metals which rust more 
evenly. A rough analogy may be drawn by imagin- 
ing an imperfect mixture of black and white sand, the 
respective grains of which may lie in streaks, spots 
and layers, or may tend to arrange themselves in some 
more or less uniform relation to each other. The best 
demonstration that the rusting and corrosion of iron 
and steel in all its forms is essentially an electrolytic 
phenomenon is afforded by the fact that it has not 
as yet been possible to find a specimen of such purity 
that no trace of positive or negative nodes will be 
formed in the ferroxyl indicator. 

Having now outlined the principle of tho 
electrolytic theory, it will be instructive to 
consider it in its practical relation to the 
problems at hand and see if it agrees with 
observed facts. It was stated in the preced- 



36 CORROSION" OF IRON" 

ing chapter that colloidal ferric hydroxide be- 
haves like an ion and travels to the negative 
pole under the influence of a current, and 
since a current must flow, according to the 
theory, when iron corrodes we should expect 
the ferric hydroxide to be affected. Now 
anyone who will examine, under a glass, a 
piece of badly rusted iron will often find a 
place where the rust is piled up like a cone 
and surrounded by a ring which has been 
eaten into the metal, or perhaps, the metal 
has been dissolved out of a certain spot and 
the ferric hydroxide piled up all around it, 
like a crater. It can be assumed, therefore, 
that the cone marks a point of negative po- 
larity, while in the other case the depression 
in the centre of the ring is positive. 

It will be evident from what has gone be- 
fore that anything which tends to prevent the 
formation or flow of current also lessens the 
tendency to corrosion, a point which will be 
touched upon later in connection with insulat- 
ing and conducting pigments. So we should 
expect that very pure iron would be more re- 
sistant than that which is impure, and this is 
found to be true, at least in large measure. 
As evidence of the truth of this general prop- 
osition, it has been shown that if two nails, 



THEORIES OF CORROSION 37 

say, are placed in water or f erroxyl and one 
of them is connected to a piece of platinum 
wire while the other is left free, the former 
will corrode much more rapidly than the 
other. This may be explained by saying that 
the hydrogen which is set free can deposit 
on the wire and thus be removed from the 
scene of action much faster than in the other 
case. 

In a similar way, when two pieces of iron, 
one of which is attached to a piece of zinc, 
are immersed together in water or some elec- 
trolyte, the free strip of iron will rapidly de- 
cay, while the piece connected to the zinc will 
be protected, although the latter slowly dis- 
solves. Zinc is electro-positive to iron ; there- 
fore a current flows from it to the iron, caus- 
ing hydrogen to deposit on the latter and thus 
protecting it. (Gaseous or atomic hydrogen 
must not be confused with that which is in 
the ionic state. It has been shown that the 
latter attacks iron and takes it into solution, 
but gaseous hydrogen, settling out in the 
form of bubbles on the iron, will make a thin 
film of gas which is a good protective from 
further action. This is illustrated by the 
^^ polarizing'' of an ordinary sal ammoniac 
battery.) 



38 CORROSION- OP IRON" 

These examples help to show why zinc, as 
applied in galvanized coatings, is such a good 
protector of iron. Through its impermeabil- 
ity it keeps the iron away from harmful influ- 
ences as long as possible; then when mois- 
ture and air do reach it, an electrolysis is set 
up which effectually protects it until most of 
the zinc is destroyed. 

If a piece of iron oxide is substituted for 
the zinc in the foregoing experiment, it is 
found that the free piece of iron corrodes 
much more slowly than the one in contact 
with the oxide, showing that this must be 
electro-negative to the iron, therefore hasten- 
ing its corrosion. The coupling together of 
two parts, one of which is electro-positive to 
the other, with the result that the corrosion 
of the former is much accelerated, brings to 
mind a possibility which is ever present in 
iron structures : that is, through some cause 
or other one member may be positive or nega- 
tive to the others and, unless proper precau- 
tions are taken, all the conditions for early 
decay of some of the members are present. 
At least one instance of such action has been 
recorded, when certain members of some 
structural iron- work were almost entirely de- 
stroyed, while others were in practically per- 



THEORIES OF CORROSION" 39 

feet shape. There seems to be no doubt that 
the parts which were destroyed were electro- 
positive to the others. The obvious remedy 
for such a condition is to assemble only parts 
having practically the same chemical compo- 
sition and treatment and, further, to insulate 
them from each other as far as possible by 
coating all contact surfaces with red lead 
in oil. 

Mention has been made of the fact that 
stresses and strains, whether produced dur- 
ing the process of manufacture or afterward, 
as well as improper heat treatment and segre- 
gation of impurities, are great factors in de- 
termining the extent and magnitude of the 
differences of electrical potential which will 
be present. To these may be added the effect 
of pickling in acids, a process which is widely 
used in preparing metal parts for finishing. 
Investigation has shown that iron which has 
been pickled may contain considerable quan- 
tities of hydrogen, being made markedly 
harder thereby, and is easily oxidized. In 
the experience of the writer, pickling is dis- 
tinctly bad and should be avoided whenever 
possible, especially in cases where even slight 
traces of rust are objectionable. Of course, 
this applies only to painted or japanned 



40 CORROSION OF IRON" 

work, as all electro-plating processes demand 
some such preparation of the parts and bad 
results do not seem to follow. It is interest- 
ing to note, however, that electrolytic iron 
which contains a large amount of hydrogen 
is corroded with difficulty. Other considera- 
tions lead to the belief that the hydrogen in 
pickled iron is not present in very stable 
combination, while that in electrolytic iron 
forms a true alloy, which may be much more 
resistant than iron alone. 

Coming now to iron which is actually in 
service in buildings and similar structures, 
it is found that this often behaves in a rather 
unexpected manner as regards its tendency 
toward corrosion. Those parts immersed in 
a few inches of water may corrode with great 
rapidity, while other parts placed a consider- 
able distance under the surface will remain 
in practically perfect shape almost indefi- 
nitely. Similar conditions are found in parts 
buried in the ground. The explanation is to 
be found in the fact that both water and 
oxygen are necessary to produce corrosion. 
If, therefore, iron parts are immersed in 
water or buried in the ground to a depth 
greater than that to which oxygen can pene- 
trate, they will be preserved because the con- 



THEORIES OP CORROSION" 41 

ditions for corrosion have not been attained. 
The literature contains many examples of 
such action and it is impossible to do more 
than refer to it here. 

Somewhat along the same line is the ob- 
servation that iron in swiftly moving water 
does not corrode as rapidly as that in more 
quiet situations, dissolved oxygen being pres- 
ent in both cases. It is a matter of record 
that pipe lines corrode from the outside 
rather than from the inside. It may be con- 
jectured, on the basis of the electrolytic 
theory, that under these constantly changing 
conditions the nodes or points having a 
different potential are changed too fre- 
quently for serious rusting in one spot to 
take place. 

A little observation will show that *^busy" 
iron, as it has been termed, does not rust as 
readily as that which is not in service. Per- 
haps steel rails are the commonest example 
of this and it is well known that little or no 
corrosion is seen in lines on which the traffic 
is fast and heavy. Sang thinks that this may 
be explained by the ** assumption that vibra- 
tion causes a shedding of the rust as soon as 
it is formed on the spots not protected by 
mill scale and there is, therefore, no accelera- 



42 CORROSION" OF IRON" 

tion of the action due to the accumulation of 
spongy and electro-negative rust.'' 

It is apparent that corrosion does not al- 
ways take place with the same rapidity or 
uniformity; therefore certain processes or 
treatments may render iron less susceptible 
to decay, and the word ^ * inhibitive " has been 
used by Cushman, in the sense of limiting or 
checking it. If a piece of iron is dipped into 
concentrated nitric acid for a short time, it 
is rendered passive; that is, it will stand a 
much longer exposure to adverse conditions 
than ordinary iron, and it precipitates cop- 
per from its solutions only very slowly. Just 
what the action is may be open to question, 
but apparently there is a lowering of the 
electrical potential. This state is the abnor- 
mal or unstable one, however, so the piece 
gradually returns to the normal condition 
and increasing corrosion takes place. Chro- 
mic acid and its salts also act like nitric acid 
and can produce the passive state. Work has 
been done in the way of putting these facts to 
practical use, and it may be that some effi- 
cient process will be discovered whereby iron 
can be made much more resistant. Other 
ways of inducing passivity are by electrolyz- 
ing in distilled water or caustic potash, some 



THEORIES OF CORROSION 43 

investigators using weak and others heavy 
currents. 

We have seen that hydrogen ions are stimu- 
lators of corrosion and hydroxyl ions are in- 
hibitors, so strong alkalis are perfect rust 
preventives. The explanation is that neither 
hydrogen ions nor hydroxyl ions can exist 
in the same solution in excess, in relation to 
each other. Now, any condition which tends 
to further the escape of gaseous hydrogen 
and thus eliminate its polarizing and damp- 
ing effect on corrosion acts as an aid to this 
process, so it has been found that under cer- 
tain conditions a paint film may actually in- 
crease, rather than diminish it. Walker men- 
tions a case where the application of a high- 
grade varnish to the inside of tinned-iron 
fruit cans led to highly stimulated corrosion. 
This was explained on the basis that the coat- 
ing, which was probably perfect when made, 
was ruptured and destroyed at certain points 
during the process of manufacture of the 
cans. When the fruit juices set up electro- 
lytic action the varnish acted as a depolarizer 
and combined with the hydrogen, thus stimu- 
lating corrosion to the point of destruction. 
Thus it is that a film of linseed oil alone may 
encourage rather than prevent decay, since 



44 CORKOSION OF IRON" 

it is porous and unsaturated chemically. It 
has often been the custom in the past to coat 
structural steel work with linseed oil before 
it left the shop, and many cases of rapid cor- 
rosion can be traced to this practice. Direct 
experiments have shown that iron parts 
which have been coated with linseed oil and 
the film then destroyed at certain spots by 
abrasion, corrode much faster than ordina- 
rily. This is probably due to the fact that the 
film of oil absorbs the hydrogen as fast as it 
is formed and thus acts as a depolarizer. If 
pigments are mixed with the oil, however, it 
loses this power to a great extent. A paint, 
therefore, in order to accomplish the purpose 
for which it was designed must be free from 
voids and of such a character that it will not 
act as a depolarizer. 

Another source of corrosion which must 
not be lost sight of is in wounds, indentations, 
and general roughness of the surface. Such 
peculiarities are almost always electro-posi- 
tive to the adjacent parts and deep pitting 
is produced. For this reason razors and 
other articles which have a high polish will, 
with the most ordinary care, resist corrosion 
and remain in good condition for many years, 
long after a rough piece would be destroyed. 



THEORIES OF CORROSION" 45 

It seems to be a principle, therefore, that the 
possession of a uniform surface, from the 
standpoint of chemical composition and free- 
dom from strains as well as the removal of 
all irregularities and the production of a 
more or less high polish, tends to make the 
part much less susceptible to the attack of 
corrosive influences than is the case with less 
carefully made articles. 



Chapter III 
PEOTECTIYE MEASUEES 

A FTER a workiiig knowledge of the causes 
^ ^ and general nature of corrosion is ob- 
tained, the next step is to apply it toward 
the protection of iron and steel from those 
malignant influences so familiar to every en- 
gineer. Since these materials can not stand 
exposure very long, while unprotected, it is 
evident that they must be covered with some 
substance or preparation which will exclude 
moisture and air and itself be impervious to 
the action of these agents. We may say that 
this represents the ideal and that in practice 
it is almost, if not absolutely, imj)ossible to 
attain these conditions completely, although 
improvements are constantly being made in 
consequence of the great amount of work 
which is being done along these lines. 

Eoughly, the problem of the protection of 
iron and steel falls into three classes: 1. 
Protection by a coating of oxide produced on 
the surface by various processes. 2. Protect- 
ee 



PROTECTIVE MEASURES 47 

ive coatings of other metals. 3. Protect- 
ive coatings formed by paints and similar 
materials. 

Many years ago Lavoisier, a famous 
French chemist, noted the formation of mill 
scale, which is mostly the magnetic oxide of 
iron, and commented on its stability and im- 
pervious character. Later on Faraday de- 
scribed the protective action of the oxide 
formed in the tubes of a steam superheater. 
Another observer discovered its formation 
when iron is acted on by highly heated air, 
and Prof. BarfP, of London, became inter- 
ested in this oxide as a protective measure 
through his observations of a pipe carrying 
superheated steam. His process, which was 
first made known in 1876, consisted in heating 
the parts to 1,000 degrees C. and passing 
steam superheated to 538 degrees C over 
them. 

A Mr. Bower and his son tried using air 
instead of steam, but were unable to obtain 
the desired results until they employed pro- 
ducer gas to reduce the red oxide formed by 
heating the parts in air. The air treatment 
lasted forty minutes and the gas treatment 
twenty minutes, these being repeated alter- 
nately four to eight times. Finally they pur- 



^8 COREOSION OF IRON" 

chased tlie Barff patent and the Bower-Barff 
process was originated. 

In this process the parts to be treated are 
heated to nearly 900 degrees C. in a closed 
retort; superheated steam is then led in for 
twenty minutes and a coating consisting of a 
mixture of the black and red oxides of iron is 
formed. Producer gas is now substituted for 
the steam and passed in for about the same 
length of time. If the coating thus formed 
is not sufficiently thick, these operations are 
repeated as often as necessary. Paraffine or 
some other oil is afterward applied to the 
parts, whereby a fine black color is obtained 
and additional protection afforded. 

As regards cost, this process is expensive, 
the price ranging from $5 to $20 per ton, but 
it can be applied to almost anything which 
will not be injured by the heat and is small 
enough to be placed in the furnace, and it af- 
fords very efficient protection, especially 
against sea-water, acid fumes, and like in- 
fluences. Paints and enamels can be applied 
and adhere strongly. Parts given this treat- 
ment will stand almost any degree of heat 
without injury, but they may not be bent or 
machined. Of course, the process is not ap- 
plicable to tools or tempered pieces. Also, 



PROTECTIVE MEASURES 49 

the parts are enlarged slightly so that those 
which must maintain close limits on dimen- 
sions should not be given this treatment. 

An improvement introduced later into 
American practice, known as the Wells proc- 
ess, consists in finishing the work at one op- 
eration, by using steam and producer gas 
together instead of applying them alter- 
nately. 

The Gesner process, which is a modification 
of the preceding, came into use a few years 
later. The coating is said to be a compound 
of hydrogen, iron, and carbon, and has less 
tendency to scale. Analyses have shown that 
not less than 2 per cent of hydrogen is pres- 
ent. The method of application consists in 
keeping the parts in the coating retort at a 
temperature of about 600 degrees C. for 
twenty minutes, then allowing steam at low 
pressure to act at intervals for thirty-five 
minutes. It is passed through a red-hot pipe 
in the bottom of the retort and thus is par- 
tially decomposed into hydrogen and oxygen. 
After the steam treatment, a small quantity 
of naphtha or other hydrocarbon is poured in 
and allowed to act for fifteen minutes, where- 
by any red oxide is reduced and the surface 
carbonated. 



60 CORROSION^ OF IRON 

A variety of work has been finisliecl by this 
process, apparently at a somewhat lower cost 
than for Bower-Barffing. It is also claimed 
to have an advantage over the latter process 
in that the much lower heat does not burn 
or warp the parts and that little or no in- 
crease in size takes place. 

Perhaps mention should also be made of 
the Dewees-Wood process which was almost 
identical with that of Gesner and preceded it. 
In this, sheet iron was subjected to the ac- 
tion of hydrocarbon vapors, or gas, and 
superheated steam in a closed, heated cham- 
ber. 

The Bradley process was patented in 1908 
and is similar in principle to the last two. 
In carrying it out, the parts are prepared 
by tumbling, pickling, or sand-blasting, pref- 
erably the latter, and heated to a low red in 
a muffle, hydrogen gas being passed in. This 
is followed later by small quantities of gaso- 
lene which improves the color of the coating. 
The articles are left in the furnace for an 
hour, or until a sufficiently heavy coating of 
the oxide is produced, after which they are 
taken out, allowed to cool, and treated with 
linseed oil or paraffine. 

The Bontempi process is another recent 



PROTECTIVE MEASURES SI 

modification or improvement on the original 
Bower-Barif method. It consists in heating 
the article as in the Bradley process and then 
passing in steam and the fumes of zinc or 
some heavy hydrocarbon as tar or pitch. A 
new patent has been recently granted cover- 
ing the use of certain substances which are 
claimed to further the formation of the black 
oxide. A uniform finish of a deep black color 
is obtained, which is said to resist corrosion 
indefinitely. 

Among the several methods which have 
been used for producing a black oxide coating 
by the use of moderate heat, that of Buffing- 
ton may be mentioned. Briefly, it consists in 
melting manganese dioxide and potassium 
nitrate (saltpetre) in an iron pot, immersing 
the cold parts in the mixture for a few mo- 
ments, then hanging them over the pot in 
the fumes and finally putting them in boiling 
water to cool. A fine blue to bronze color is 
produced. 

In about the same way, films of sulphide 
or phosphide of iron, produced by subjecting 
the metal to the vapors of these elements at 
high temperatures in the absence of air, have 
also been tried, but have not attained to any 
very extended use. 



52 CORKOSION" OF IRON" 

Heat-bluing, which is familiar to every one, 
is applied to watch and clock hands, buttons, 
buckles, and a large variety of steel articles, 
and gives a finish of a pleasing blue or black 
color, which is not, however, so very resistant 
to corrosion. It is produced in several ways, 
such as dipping the parts in a bath of molten 
saltpetre, heating them on an iron plate in 
the air, or tumbling them in a sheet-iron bar- 
rel heated by a gas flame. Eevolvers and 
similar parts are very often given a fine blue 
finish by heating in charcoal. 

One of the best and simplest methods of 
producing a protective black coating is of 
rather recent origin, and is known as Coslett- 
izing, from the name of the discoverer, Cos- 
lett. In this process the parts are first 
cleaned by pickling or sand-blasting, then im- 
mersed in a boiling water solution of phos- 
phoric acid in which iron or zinc filings are 
placed. They are left in this for a period 
varying from one-half hour to three hours, 
depending on the nature of the work or the 
thickness of the coating desired. A very 
slight amount of the surface of the article 
treated is converted into certain phosphates 
of iron, but most of the coating comes from 
the solution itself. One advantage of this 



PROTECTIVE MEASURES 53 

method is that it can be applied to small and 
delicate parts, tempered pieces, edged tools, 
and so on. Typewriter manufacturers and 
others who are using it claim that it retards 
corrosion to a remarkable degree, especially 
on articles which are in use in the tropics and 
other places where unusually good protection 
is required. 

In summing up all the above processes, it 
may be said that Bower-Barffing or any other 
procedure which gives a heavy coating con- 
sisting essentially of the magnetic oxide of 
iron will afford a durable, efficient finish. 
It is not absolutely rust-proof, as experience 
shows that tiny rust spots will generally de- 
velop in the course of time. Under extremely 
bad conditions these appear sooner and 
doubtless the finish would go to pieces event- 
ually; but although this coating is strongly 
electro-negative to iron, a circumstance which 
we should think would tend to accelerate cor- 
rosion rapidly when it is once injured, the 
breaking down is very slow. Perhaps the 
perfect adherence of the oxide allows the iron 
to rust only at the exposed places, so that, 
although it may pit deeply, it is not so no- 
ticeable as though it were more extensive. 
As noted, however, those methods which em- 



54 CORROSION OF IRON" 

ploy higli temperatures are not applicable to 
all classes of parts. 

Coslettizing appears to be the best of tbose 
processes employing low degrees of heat, and 
it can be applied to practically any iron or 
steel article, giving it a durable finish. It is 
not very complicated or expensive, although 
the cost is increased through the fact that 
it is a patented process ; therefore a royalty 
must be paid. 

Lastly, those methods which produce 
merely a darkening of the surface ordinarily 
do not afford much protection from rust, es- 
pecially on parts exposed to high humidity. 
They have their place, however, and when 
used in conjunction with lacquers or like ma- 
terials may be very useful in producing a 
fairly good finish. 

Coming now to those processes which de- 
posit another metal or alloy on the surface of 
iron, for the purpose of protecting it, it is 
found that those involving the use of zinc 
in some manner are by far the most impor- 
tant. Several factors tend to make this true, 
but we may mention the comparative cheap- 
ness of zinc and the fact that it is peculiarly 
suited to this work, since it is the most elec- 
tro-positive to iron, as has been mentioned, 



PROTT]CTIVE MEASURES 55 

of all the metals which can be used practically 
for such purposes. 

Unlike most metals, zinc may be applied 
to iron objects in three widely different ways : 
by dipping them in the molten metal ; by de- 
positing it electrolytically from a plating 
bath ; and by the vapor or Sherardizing proc- 
ess. All these methods are widely used, and 
each has advantages which may especially 
commend it in certain cases. 

In the hot or dip method, the parts are 
cleaned by pickling, generally dipped in some 
acid flux, and then immersed in a bath of 
molten zinc. Sal-ammoniac or other fluxing 
agent is used in addition to insure the clean 
surface requisite for perfect adherence of the 
coating. After the parts have become thor- 
oughly heated and covered with the zinc, they 
are withdrawn and the excess of metal is 
shaken or wiped off, depending on their 
shape. 

At first sight it would seem that a very 
pure coating of zinc is thus produced, but a 
little consideration will show that such is not 
the case. In the first place, the zinc bath 
is not strictly pure, but generally contains a 
fairly considerable amount of impurities in 
the shape of other metals, with perhaps slight 



56 COKKOSION OF IRON 

quantities of dissolved oxides or oxy-cMo- 
rides. Iron, too, is more or less attacked by 
molten zinc, so the galvanizing kettle and the 
parts themselves contribute a good deal of 
iron. After the iron content reaches a cer- 
tain point a definite alloy of iron and zinc 
settles out, and to prevent it from adhering 
and becoming burned onto the bottom of the 
kettle it is the practise of many foremen to 
add lead to the bath. This melts and, being 
heavier than either the zinc or the alloy, set- 
tles to the bottom and everything else rests 
on top of it. The removal of the alloy, there- 
fore, is not difficult and is done at regular in- 
tervals. In addition to the foreign sub- 
stances above mentioned, many galvanizers 
add a small amount of aluminum or tin to aid 
in the production of a smoother and better 
looking deposit. 

It is easily seen, therefore, that the coating 
actually produced is a mixture of zinc with 
a number of other metals, and perhaps some 
actively corrosive bodies derived from the 
fluxes. The resulting alloy is very easily at- 
tacked by acids, due, no doubt, to the electro- 
lytic action set up between the various met- 
als. According to some authorities the solu- 
tion tension of zinc alloy with iron and 



PROTECTIVE MEASURES 57 

other impurities is greater than that of 
the pure metal. When a hot galvanized part 
is exposed to adverse conditions, therefore, 
it is plain that unexpectedly severe corrosion 
may take place. A good deal of work is be- 
ing done to improve the quality of hot gal- 
vanized coatings, in the way of employing 
harmless fluxes and keeping the zinc as pure 
as possible, so it is likely that much progress 
will be made, although the very nature of 
the process is such that a very pure coating 
can not be produced, at least commercially. 
Nevertheless, this process is used very widely 
and enormous amounts of iron are finished 
by it. 

In using galvanized or other material 
which has been given a protective finish, it 
is necessary to have some means of determin- 
ing the probable efficiency of the coating as a 
preventive of rust, and this is done indirectly 
by measuring its weight and thiclmess. For 
anyone who has access to a good balance and 
a few chemicals, perhaps the simplest way is 
to clean and weigh a small piece of the ma- 
terial, and then boil it in strong caustic soda 
until the zinc is all removed. The piece is 
then washed, dried, and weighed again. Sub- 
tracting this weight from the former one 



58 CORROSION" OP IRON 

gives the weight of zinc present, and by divid- 
ing it by the area of the part in square inches 
the result can be expressed as milligrams per 
square inches or ounces per square foot. It 
is not always practical or possible, however, 
to inspect material in this way, so another 
method known as the Preece test is in wide 
use. 

This depends on the fact that zinc, when 
dipped into a strong solution of a copper salt, 
is dissolved and takes the place of the copper, 
which appears in metallic form. Iron does 
the same thing as the zinc, but much more 
slowly, so that the copper deposit formed on 
the zinc is very coarse and easily removed 
while that on the iron is more or less hard 
and bright. Putting these together, when a 
galvanized piece is dipped in copper- sulphate 
solution, the copper first deposited is easily 
wiped off, but as soon as the zinc is all gone 
the copper plates out on the iron, this point 
being determined by the difference in ap- 
pearance and behavior of the coating. Prac- 
tically, it is sometimes a little difficult to tell 
just when all of the zinc is removed; also 
there are many factors which may work to 
impair the value of the results obtained, so 
exact directions have been drawn up cover- 



PROTECTIVE MEASURES 59 

ing tlie application of the test and the means 
by which the critical point may be recognized. 
Thus there is, ordinarily, a good degree of 
uniformity in the manner of using it. For ex- 
ample, the American Steel and Wire Co. di- 
rects that the copper solution shall be neu- 
tralized by an excess of pure copper oxide, 
it shall be filtered before using, and shall 
have a specific gravity of 1.186 at 65 de- 
grees F. 

Samples must be thoroughly cleaned of all 
grease and dirt and the temperature of the 
solution must be not lower than 65 degrees 
F. nor more than 70 degrees F. In making 
the test the carefully prepared samples are 
immersed in the fresh solution for exactly one 
minute. They are then taken out, rinsed in 
water, and wiped with clean cotton waste, 
after which they are immersed again for ex- 
actly one minute, washed and dried. If the 
material is required to stand more than two 
dips, the process is repeated the requisite 
number of times; then the parts are care- 
fully examined to see if there are any traces 
of bright copper. If no deposit appears, the 
parts are considered to have passed the test. 

Although this procedure finds wide appli- 
cation, there are several disadvantages or 



60 CORROSION OF IRON" 

faults in it, to which attention has been called 
by investigators. These will be referred to 
later, after the other methods of producing 
zinc coatings have been considered, inasmuch 
as this test has been applied to all these 
indiscriminately. 

It is only within comparatively recent 
years that it has been possible to obtain a 
good deposit of zinc electrolytically, on a 
commercial scale. A great deal of work has 
been done along these lines, however, so that 
we are now able to produce very fair coat- 
ings, although there is still plenty of room 
for improvement. In brief, the process con- 
sists in immersing the carefully pickled and 
cleaned parts in a solution composed essen- 
tially of zinc sulphate, with a very slight 
amount of free sulphuric acid. Various or- 
ganic substances are also added to assist in 
the production of a smooth, dense deposit. 
There is a large number of formulae for these 
baths, many of which are patented or secret. 
The plating time varies from twenty minutes 
to an hour or more, depending on the current 
density, the weight of deposit required, and 
so on. Some of the solutions permit the use 
of current densities as high as fifty amperes 
per square foot, which is much higher than 



PROTECTIVE MEASURES 61 

was possible when the process first came into 
use. After the plating period is over, the 
parts which have been plated are washed in 
water and dried. 

The coating which is produced is of great 
purity; therefore, it would seem to be su- 
perior, in this respect at least, to that given 
by hot galvanizing. When properly carried 
out, a very smooth, uniform deposit is ap- 
plied, so this method is applicable in many 
cases, as with small parts, where the hot proc- 
ess would clog up small holes or threads 
and prove generally unsatisfactory. It also 
possesses the great advantage of very easy 
control — that is, a coating of definite weight 
can be produced at will. 

On the other hand, the deposit is more or 
less porous, depending on the conditions un- 
der which it is made, and certain impurities 
may be included which will become active 
stimulators of corrosion. As far as protec- 
tion is concerned, there does not seem to be 
much to choose between the hot dip and elec- 
tro methods, when testing parts of the same 
weight of deposit. 

As before, the actual weight of zinc per 
unit area may be determined by stripping 
in caustic soda, weighing the parts before 



62 C0EE0SI02T OP lEON 

and after stripping, and dividing the differ- 
ence in weight by the area of the part. 

In testing electro-galvanized work to see 
if the coating is of the desired weight, sam- 
ples are often suspended in distilled water 
through which air is bubbled, the time re- 
quired for the first sign of rust to appear be- 
ing carefully noted. Under such conditions 
the zinc is converted to the hydroxide or basic 
carbonate and settles to the bottom of the 
vessel, exposing the iron. In making the 
test it is important that certain definite con- 
ditions be always maintained, especially as 
regards the rate at which the air is passed, 
if comparative results are to be obtained. In 
passing, it may be said that the value of such 
an acceleration test, as a measure of the 
efficiency of a coating in protecting iron from 
corrosion, may be a doubtful quantity but it 
does roughly indicate the weight of the coat- 
ing. The Preece test is also applied to elec- 
tro-galvanized work, and when properly car- 
ried out should be of great value in determin- 
ing the quality of such a finish. 

Sherardizing was developed by Sherard 
Cowper-Coles, although some of the funda- 
mental facts regarding it have been known 
for a long time. In this process the thor- 



PROTECTIVE MEASURES 63 

oughly cleaned parts are placed in an iron 
drum with a quantity of finely powdered 
zinc, which generally contains a considerable 
amount of zinc oxide. This drum is then 
placed in a gas-fired or electrically heated 
furnace and brought to a temperature of 
about 800 degrees F. which is maintained 
for a varying length of time, depending on 
the thickness of coating desired. On the 
average a furnace will turn out two charges 
a day. When the drums are sufficiently cool 
they are emptied over a grid or screen so 
that the zinc dust falls through and collects 
in bins, to be used again. 

One great advantage of this process is the 
fact that the zinc coating is applied very 
evenly and uniformly over all surfaces ac- 
cessible to the powder. In this respect it is 
superior to the methods previously consid- 
ered, especially the electrolytic, with which 
difficulty is experienced in getting the zinc 
to deposit in inner corners and recesses. The 
size of the parts is increased very slightly by 
this treatment, but perhaps the main action 
is one of alloying with the iron rather than 
depositing zinc on the surface. Dr. Walker 
says that a polished and etched specimen 
of Sherardized iron presents a relatively 



64 COEEOSIOIT OF IRON" 

complex structure. The zinc penetrates the 
iron with the formation of deep layers of 
two alloys B and C, the latter being richer 
in iron than the other. Imposed on B there 
are a number of more or less unknown alloys 
containing varying amounts of zinc and iron. 
Upon the surface there is generally a layer 
of relatively pure zinc, although frequently 
the process is carried to the point where only 
a deep layer of alloys is formed. When ex- 
amined under a microscope this alloy is seen 
to be covered with cracks or fissures, as 
though it had contracted in forming. The 
claim is made that even if the outside coat- 
ing, which is rather brittle, is removed by 
blows or rough usage, the part will still be 
protected by the deeper zinc-iron alloys and 
will not rust, and practical experience seems 
to bear this out. Further, if a deep notch 
is filed in a piece of Sherardized work and 
the latter is then exposed to the weather or 
placed in aerated water, rust will form in 
the notch and fill it up completely, then stop. 
It apparently can not crawl or loosen the 
surrounding coating. 

When parts finished by this process are 
exposed to the weather or immersed in water, 
a yellowish-brown color which may easily be 



PROTECTIVE MEASURES 65 

mistaken for rust develops. This is called 
the curing color and is probably caused by 
the corrosion of part of the iron in some of 
the alloys. At any rate the surface rapidly 
turns darker and soon becomes almost black, 
from the formation of the magnetic oxide 
which is in itself a good protector. On con- 
tinued immersion a few weak spots will gen- 
erally show up and tiny rust spots form, but 
as remarked, these do not spread and for 
all practical purposes may be disregarded. 
Altogether, the work produced by this method 
seems to be as nearly rust-proof and durable 
as can be produced by commercial means at 
the present time. The fact that it is adapted 
to the finishing of both large and small parts, 
even those having threads, as screws or pipe, 
greatly increases its applicability and it is 
being used very widely. 

In the ^* molten zinc'^ process the parts are 
placed in a hollow drum which is placed in- 
side a larger one. The zinc is heated to the 
volatilizing temperature, while hydrogen or 
other reducing gas is forced in to prevent 
it from oxidizing, and it combines with the 
iron, producing about the same results as 
Sherardizing. Aside from its use as a means 
of affording protection, this method is also 



66 CORROSION OF IRON 

capable of application in decorative work. It 
is possible to get very beautiful effects by 
forming alloys with other metals — for ex- 
ample, the production of a brass design on a 
copper background. 

Another procedure, of which only the bar- 
est mention can be made, has been developed 
for finishing wire. It consists in passing a 
heavy current through the wire, thereby 
heating it to redness, and drawing it through 
a mixture of crushed coal and zinc oxide 
in a furnace. The heat reduces the oxide 
to the metal, which then alloys with the iron 
of the wire, forming a coating similar to 
Sherardizing. Whether or not this process 
will come into very extended use remains 
to be seen. 

Some little reference has already been 
made to the Preece test for galvanized work. 
As a whole it has been objected to on the 
ground that it assumes that the zinc is re- 
moved in direct proportion to the time of im- 
mersion and that it really determines the 
thickness of the coating at its thinnest part. 
When applied to electro-galvanized or hot 
dipped work it is a fairly satisfactory means 
of indicating the thickness and uniformity 
of the coating, and meets commercial require- 



PEOTEOTIVE MEASURES 67 

ments of speed and reliability. When ap- 
plied to Sherardized material, however, it is 
the opinion of many investigators that very 
misleading results may be obtained, in spite 
of the fact that some manufacturers are us- 
ing it. The trouble apparently is that some 
of the zinc-iron alloys are indicated as iron, 
while in reality they have very good pro- 
tective powers. Perhaps if carried out un- 
der very definite conditions and making cer- 
tain allowances it would prove of value as a 
ready test, but there are other means of de- 
termining the quality of the coating and these 
are much more accurate, though longer. A 
very good way is to strip a piece in caustic 
soda, as described before. Immersion in 
water is also a reliable means of testing all 
such coatings. One of the largest producers 
of Sherardized material in the country is 
said to test all of its product by subjecting 
samples to a strong salt-water spray for 
something like sixty hours. 

It should be remembered that all the fore- 
going methods of testing in which the weight 
of the deposit is determined do not give much 
information regarding the freedom from 
pores. Walker therefore devised a scheme 
of testing electro- and hot-galvanized ware 



68 CORROSION OF IRON 

for pinholes in the coating by immersing the 
part in strong, hot caustic soda. Any such 
imperfections will then be revealed by the 
formation of streams of small gas bubbles. 

Short and simple tests of quality are a 
necessity and are of real value when rightly 
used, but the best method of all is by actual 
exposure to the weather. It requires con- 
siderable time to obtain definite results, but 
when it is possible to do this it is generally 
worth the time and trouble, as it decides im- 
partially and unmistakably between various 
finishes or different samples of the same 
finish. 

Of the other metallic coatings produced 
by electro-plating, perhaps copper and nickel 
are the most familiar. These are applied 
both for decorative purposes and to afford 
protection from corrosion, and are used very 
widely for finishing an infinite variety of 
small and medium sized parts, although there 
are certain disadvantages connected with the 
use of such coatings. The processes for pro- 
ducing them are rather expensive and call for 
a high degree of skill; to be really effective 
under bad conditions a heavy deposit is re- 
quired, and even then there is a decided ten- 
dency to wear away, especially on edges, if 



PROTECTIVE MEASURES 69 

the parts are subjected to much handling. 
For many purposes, however, no better 
methods of protection have been devised. 

Copper is deposited from two kinds of 
baths, acid and cyanide. The former is es- 
sentially a solution of copper sulphate with a 
small amount of free sulphuric acid, and is 
used for plating on any metal which is not 
attacked by dilute sulphuric acid. Iron, for 
example, can not be plated in it because it 
reduces a copper solution, as we have seen, 
and is itself dissolved by the acid, so the cop- 
per cannot adhere to it and the cyanide solu- 
tion must be resorted to. This is made by 
dissolving basic copper carbonate in a slight 
excess of potassium cyanide. It does not at- 
tack iron or other metals to any extent and 
rapidly produces a fine deposit. 

Nickel solutions are generally made from 
nickel sulphate; some platers use the double 
nickel-ammonium salt, a small amount of free 
acid being present in both cases. Boric acid 
is added to brighten the deposit. 

Among the metallic coatings produced in 
other ways, mention may be made of copper- 
clad steel, which seems to be finding a con- 
siderable field of usefulness, perhaps espe- 
cially in the form of wire. Space does not 



70 COREOSIOX OF IRON" 

permit a description of the process beyond 
saying that the copper is applied in one of 
two ways, by alloying or by welding. In the 
former the copper is in the plastic state, 
while in the other it is molten. Copper is 
very resistant to atmospheric influences, and 
exposure tests of copper-clad wire have 
shown it to be practically unchanged after 
long periods. 

We should naturally expect that the con- 
tact of steel with copper would cause the 
former to corrode very rapidly, but this 
does not seem to be borne out by experiment 
and it has been conjectured that after a cer- 
tain amount of rust has formed a thin film of 
copper oxide mixed with copper is deposited 
between the iron oxide and the unattacked 
iron and that this acts as a preservative coat. 
Altogether it would seem that such copper 
and iron combinations should come into wide 
use, especially as the former metal is not 
destroyed by exposure to moisture and air as 
is zinc. 

Lead has been used to some extent, and 
the Lohmannizing process, which is perhaps 
the best known of these, is said to consist 
in the immersion of the parts in a bath of 
molten pure lead or lead alloys, the secret 



PROTECTIVE MEASURES 71 

being in the use of the proper fluxes. While 
an alloy of lead and tin is used in the manu- 
facture of terne plate, it is somewhat doubt- 
ful if the use of pure lead in this way will 
come to be very extensive. 

Another procedure, which differs consider- 
ably from any of those so far discussed and 
presents some novel features, is that known 
as the Schoop process, wherein some metal 
is sprayed by means of an air blast onto the 
parts which it is desired to protect. The 
apparatus for doing this is more simple than 
might be supposed and consists of a sort of 
pistol, in appearance not unlike an automatic 
revolver, having three hose leads, through 
which are conducted oxygen, hydrogen, and 
compressed air. Inside the pistol the two 
gases are burned in a burner tube concentric 
to the air nozzle. 

Metal is supplied in the form of a wire, 
which is drawn in by a suitable feeding de- 
vice actuated by a small air turbine in the 
body of the pistol. When the wire reaches 
the flame zone the intensely hot oxygen- 
hydrogen flame instantly melts it, while at 
the same time a strong blast of air issues 
from the nozzle and blows the molten metal 
into a cloud or spray, so that it is discharged 



72 corrosio:n" of iron 

from the pistol with a velocity said to be 
approximately 3,000 feet per second- 
It might appear at first sight that the metal 
would reach its mark in a molten state, but 
this does not seem to be the case since a 
few inches from the nozzle it is of fairly low 
temperature, low enough to be directed upon 
the hand for a moment, or upon silk or other 
cloth, or even matches. Perhaps the tiny 
particles are really solid when they strike 
the part under treatment, but are liquefied by 
the heat of the collision. Whatever the ac- 
tion may be, however, the fact remains that 
a dense, strongly coherent coating is pro- 
duced, which is said to be amorphous and 
vitreous, rather than crystalline. 

Any desired weight or thickness of deposit 
may be obtained at will and almost any metal 
or alloy can be used, so the process would 
seem to have a wide field of usefulness be- 
fore it, both from protective and decorative 
standpoints. While it is yet comparatively 
new and has not come into extended use, 
there does not seem to be any good reason 
why it could not be employed to coat bridges 
or other forms of structure work, either 
wholly or at any desired point, as the neces- 
sary apparatus is not very cumbersome or 



PROTECTIVE MEASURES 73 

expensive. It is claimed that lead can be 
sprayed for less than two cents per square 
foot and one pound can be deposited in less 
than one minute. 

Various other alloys and processes are be- 
ing proposed from time to time and doubt- 
less some of them will prove to be of real 
value, but so far as the writer is aware none 
of them has displaced to any great extent 
the older protective methods. 

The protection of iron and steel by paints 
and similar materials will be considered in 
succeeding chapters. 



Chaptee IV 
PAINT MATEEIALS 

TN the preceding chapter the most impor- 
tant protective coatings for iron and 
steel (other than those produced by paint and 
similar substances) were considered, and now 
we come to the study of these latter ma- 
terials. 

Galvanizing, plating, and the other methods 
which have been discussed are very widely 
used and are of the greatest value, but the 
use of paint is hardly secondary to these 
and is worthy of the deepest attention. Per- 
haps one reason why this is true is that the 
plating methods previously referred to are 
not always applicable, for some cause or 
other. For example, it would be impractica- 
ble, both from cost and manufacturing stand- 
points, to galvanize or copper-plate the 
mighty members of a railroad bridge or simi- 
lar structure. Also, in much of the piping, 
railing, machinery, and other iron work of 
everyday life, it would be altogether too ex- 

74 



PAINT MATERIALS 75 

pensive to apply a plated finisli, and even if 
this was done it would be necessary to use a 
finishing coat of some other material, if only 
for the sake of appearance. In other words, 
the methods previously described are limited 
in their applicability by their cost and the 
size of the parts which can be treated. In 
all such cases, therefore, recourse must be 
had to some form of paint. 

Everyone knows, in a general way, that 
paint is composed of certain pigments sus- 
pended in linseed oil, with perhaps the addi- 
tion of a drier and some turpentine or ben- 
zine, and that it is put on with a brush. In 
order to study paint intelligently, however, 
it will be necessary to review some of the 
facts regarding its manufacture and the in- 
gredients composing it. 

In general, there are two ways of making 
paint. In the first, the pigment is ground in 
a mill with only enough linseed oil to form 
a stiff paste, which is thinned when needed 
for use by mixing with more oil, turpentine, 
and drier. A little varnish may also be 
added to improve the quality of the film. In 
the other method, the pigment is thoroughly 
mixed with the amount of oil necessary to 
make a paint of the desired consistency, and 



76 coKEOsiox or ieon 

tMs mixture is tlien gronnd by numiiLg it 
thi'ongh a paint mill several times. 

Both of these procedures have certain ad- 
vantages, but it is doubtful if the pigment 
and oil can ever be as thoroughly incoi'po- 
rated, when the first method is used, as when 
they are ground together: so vrhen the best 
results are expected it is better to obtain the 
paint ready mixed from the factory. An- 
other point in favor of this is that such a 
paint is far more likely to have all the ingre- 
dients mixed in the right proportions. 

For everyone who uses paint, and espe- 
cially for engineers who may be called upon 
to di'aw up specifications or make recom- 
mendations regarding it, some knowledge of 
the materials employed in paint making and 
the processes by which they are produced 
is necessary, and the principal ingi'edients 
found in the ordinaiy paints used on metal 
work will be briefly considered. 

The one substance which is common to all 
paints is linseed oil. This is obtained either 
by grinding and pressing the seeds of the 
flax jilant or by extracting them with naphtha. 
The pressed oil contains a good deal of for- 
eign matter, ^^foots,'' which may be partly 
removed bv filtration, the rest settlino; out on 



PAINT MATERIALS 77 

long standing. If the oil is painted upon a 
strip of glass or other surface and exposed 
to the air, it will slowly absorb oxygen and 
become converted into a tough film. With 
raw oil this forms too slowly, however, for 
practical purposes, so recourse is had to the 
process known as boiling, whereby the drying 
properties of the oil are greatly increased. 
Heating it to a temperature of 400 degrees 
F. to 500 degrees F. has the effect of making 
it considerably darker and more easily dried. 
The addition of salts of lead or manganese to 
oil heated to a high temperature also pro- 
duces this effect, and this was the old method 
of boiling oil. A very dark oil, however, is 
objectionable for many purposes and it is 
now the practice, at least at many places, 
to make a drier of the metallic oxides by heat- 
ing them at high temperatures with a small 
quantity of the oil, and then adding the 
residue, dissolved in benzine or turpentine, to 
the main body of oil kept somewhat above 
the boiling point of water. By this method 
fuel, time, and oil are saved and the result- 
ing product is lighter in color than with the 
other processes. 

Regarding the quality of the oil thus pro- 
duced there seems to be a considerable differ- 



78 CORROSION- OF IRON" 

ence of opinion. Some people assert it is 
as good in every way as that which has been 
heated to a high degree with the salts or 
oxides; others maintain that an inferior 
grade of oil is produced. Amid these con- 
flicting views it is hard to decide, but it is 
probable that when it is made into a properly 
designed paint it will give satisfactory serv- 
ice, at least in many cases. 

In making up a paint a certain percentage 
of raw oil is generally added to the boiled 
oil, or raw oil may be used alone, a small 
quantity of a japan or oil drier being added 
in each case. Japan driers are made by melt- 
ing resins with salts and oxides of lead and 
manganese or other metals, afterward thin- 
ning the mass with turpentine or light min- 
eral oils. The action of such a drier is very 
rapid. Oil driers are made, as noted be- 
fore, by heating linseed oil with metallic salts 
or oxides to a high temperature and thinning 
with more oil and volatile solvent. 

The oxides of lead and manganese are most 
widely used in the making of driers. Man- 
ganese salts start the drying action and cause 
the surface to dry quickly. Lead, on the 
other hand, causes the oxidation to proceed 
through the film and is generally used in the 



PAINT MATERIALS 79 

larger proportion. Red lead makes paint 
films brittle; litharge tends to make them 
very elastic. 

A raw linseed-oil paint dries so slowly that 
it is impractical and not suited for ordinary 
use, although it would produce an excellent 
film in time. The addition of a certain 
amount of drier is justifiable and necessary, 
therefore, but it must be borne in mind that 
at least some action continues long after the 
oil is dry and eventually tends to bring about 
the destruction of the film. The kind and 
amount of drier used, accordingly, has a 
great influence on the life and durability of 
the paint and close attention should be paid 
to this point. It is poor economy to sacrifice 
a year or two of the usefulness of the paint 
film in order to make it dry a few hours 
sooner. On the whole, the oil driers do the 
least harm, but even they should be used in 
moderation. 

Since the tests for the purity of linseed 
oil are to be found in many text and refer- 
ence books they will not be given here. Ac- 
cording to price conditions and so on, min- 
eral oils, turpentine, rosin and rosin oil, and 
corn, fish and cottonseed oils may be used 
to adulterate it. The detection and estima- 



80 CORROSIOJ^ OF IKON" 

tion of these is the work of the analytical 
chemist and further consideration of them 
would be out of place here. 

Among the other oils which may be added 
to, or used as substitutes for, linseed, per- 
haps tung or China wood oil is the most im- 
portant. When properly boiled and treated 
it yields a film which is hard and elastic, with 
heavy body and high gloss. One great ad- 
vantage of this oil is that it forms paints 
which will dry in damp atmospheres. It has 
long been used by the Chinese and Japanese 
and is finding wide use for marine and water- 
proof paints, and there is no apparent reason 
why it should not be used more extensively 
for protective paints for iron and steel. 

Soya bean oil is obtained from the soya 
bean which grows in Manchuria, although 
it will also grow well in many parts of the 
United States. It is very similar to linseed 
oil, so much so that it is difficult to detect its 
presence in the latter. Efforts are being 
made, and with success, to utilize it in the 
manufacture of paint. 

Menhaden or fish oil will be mentioned, as 
it is used in special paints which are required 
to stand heat and light, as on smoke-stacks. 
Such a paint is considerably more expensive 



PAINT MATERIALS 81 

than a linseed-oil paint, so it can be seen that 
menhaden oil is a valuable aid and not an 
adulterant, as often claimed. Proper treat- 
ment of the oil is very expensive, as the loss 
by evaporation is large and certain volatile 
products are formed which are very offensive 
to the workmen. Mixed with linseed oil, it is 
used to some extent for making water-proof 
paints for various purposes and seems to 
give good satisfaction. 

Coming to the materials used for thinning 
and diluting paints, turpentine is the most 
valuable. It is the product of the sap of 
several species of pine trees growing in the 
South. The sap is collected and distilled with 
steam, the turpentine passing over and the 
rosin being left behind. Foreign turpentines 
differ somewhat in specific gravity and odor, 
but have the same composition. 

When added to paint, turpentine causes the 
latter to work more easily and increases the 
spreading power. It evaporates slowly and 
completely without leaving a stain, and causes 
more raj)id oxidation of the paint oils. The 
one objection to turpentine is its compara- 
tively high price, and many substitutes are 
on the market, as such or as adulterants. 
Some of the best of these are light mineral 



SZ CORROSION OP IRON 

oils derived from the refining of petroleum 
and other substances. In their flash and boil- 
ing points they resemble turpentine, possess 
the same flowing qualities, and apparently 
are just as good. At any rate they are cheaper 
and are being used widely. One of these, 
benzol, may be especially mentioned as it is 
used very largely in coal-tar and asphaltum 
paints and in paint and varnish removers. 

Experience has shown that the value and 
durability of any paint depends in large 
measure on the pigments used, and this is 
especially true of protective paints for iron 
and steel. It has been known for many years 
that certain pigments give better results than 
others, but it is only recently that efforts 
have been made to determine the action of 
some of these materials. Considerable work 
has been done on this subject and it has been 
possible to classify pigments as stimulators, 
inhibitors, and inerts or indeterminate s, on 
the basis of extended researches carried out 
by several investigators. Finally the Ameri- 
can Society for Testing Materials decided to 
undertake a similar series of tests and the re- 
sults were so interesting and instructive that 
they are given on pages 84 to 86. Briefly, 
the method of conducting the test was to 



PAINT MATERIALS 83 

place pieces of carefully weighed steel in a 
series of bottles, each of which contained a 
pigment suspended in distilled water through 
which air was bubbled. After about three 
weeks the steel pieces were cleaned, dried, 
and weighed, the loss in weight being care- 
fully noted. In performing this experiment 
several investigators worked independently 
and these results are taken from the Proceed- 
ings of The American Society for Testing 
Materials, Vol. 10, 1910. 

In considering these results it must be 
borne in mind that they are merely indica- 
tive of the general nature of the pigments 
when suspended in water, and it does not 
necessarily follow that oil paints made from 
these would show up in service in exact agree- 
ment with the recorded test. A pigment sus- 
pended in a paint film is under vastly dif- 
ferent conditions from one in water, but 
the results of practical tests are in broad 
accord with the tabulated data, and it ap- 
pears that those pigments which, in water 
suspension, depend upon the inhibitive action 
of chromate salts or the presence of hy- 
droxylions, through hydrolysis, will, in gen- 
eral, give a good account of themselves when 
made into an oil paint. 



84 



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80 



CORROSION OF IRON 




N CS3 N N 



PAINT MATERIALS 87 

Eegarding the method of manufacture of 
some of these materials, zinc chromate is a 
yellow pigment made from zinc salts and 
potassium dichromate. It is fairly soluble 
in water and generally contains other chro- 
mates and zinc oxide, with some impurities. 
It has a specific gravity of 3.5 and grinds to 
a paste in 25 per cent of oil. Considerable 
drier is required as it is a slow drying ma- 
terial, but it has proved to be one of the 
most inhibitive of all pigments in use and is 
valuable in even small amounts in protective 
paints. Its rather high price is its only dis- 
advantage. 

Zinc and barium chromate is made by pre- 
cipitating a solution of the chlorides of these 
metals by sodium chromate. It is less soluble 
than the zinc salt and is a very good inhibi- 
tive. 

Zinc oxide is one of the most valuable white 
pigments and is produced by roasting and 
subliming certain ores of zinc. It is very 
opaque, has excellent spreading qualities, and 
is generally mixed with basic carbonate of 
lead — white lead — for use in white paints. By 
thus combining these, certain undesirable 
qualities of both are overcome. For instance, 
zinc oxide dries to a hard surface which does 



88 COEEOSION OF IRON 

not stand changes of temperature as well 
as the softer lead pigment. On the other 
hand it tends to eliminate the tendency to- 
ward chalking which the lead possesses. It 
is widely used as a base for delicate colors 
and in enamels. As has been shown, it is a 
fairly good inhibitor. 

Zinc-lead white is made from zinc-lead ores 
containing sulphur. The metals are vola- 
tilized and oxidized, forming a white smoke 
or fume composed of about equal parts of 
zinc oxide and lead sulphate, which is col- 
lected in large bags. When placed on the 
market it is extremely fine, has a specific 
gravity of 4.4 and grinds in 12 per cent of 
oil. It is often mixed with white lead and 
zinc oxide and inert pigments, is very stable 
chemically, and generally possesses inhibitive 
qualities. 

Barium chromate is pale yellow in color 
and is made by treating barium chloride with 
sodium chromate. Its inhibitive value is not 
particularly good, probably because of im- 
purities. 

Ultramarine blue is made by calcining 
silica, China clay, soda ash, and sulphur in 
furnaces and grinding the product. It is of 
a bright blue color, has a specific gravity of 



PAINT MATERIALS 89 

2.4, and grinds in 30 per cent of oil. It tends 
to turn darker on iron on account of the for- 
mation of sulphide of iron, from the sulphur 
in it, and is not considered a good inhibitive. 

Chrome green (blue tone) is made from 
lead nitrate, sodium chromate, and sulphuric 
acid, precipitated on white lead and Chinese 
blue. It contains a variety of impurities, 
whereby its protective powers are lowered, 
is a fairly heavy pigment, and grinds in 25 
per cent of oil. 

Prussian blue is made by mixing solutions 
of iron sulphate and potassium ferricyanide 
and oxidizing the resulting precipitate. It is 
a pretty blue, has a specific gravity of 1.9, 
and grinds in 55 per cent of oil. It may or 
may not be inhibitive in character, depending 
on its freedom from impurities; therefore 
it should be tested before use. It exerts a 
marked preservative action on the oil with 
which it is combined so that a glossy surface 
is presented even after long exposure. 

Lithopone is produced by mixing solutions 
of zinc sulphate and barium sulphide The 
resulting precipitate consists of zinc sulphide 
and barium sulphate ; it is filtered oif , heated 
to a high temperature, and dropped into 
water, whereby it is thoroughly disinte- 



90 CORROSION" OP IRON" 

grated. It is washed and filtered again, dried 
and ground. All of these operations require 
a good deal of care and attention to produce 
a satisfactory pigment. 

Lithopone is very stable and is probably 
the whitest pigment known, being very widely 
used in high-grade enamels. Exposed to 
light and dampness it darkens, but will often 
regain its whiteness again. It is, therefore, 
not suited to outside use, by itself, but needs 
to have a large amount of some stable pig- 
ment combined with it. It has a specific grav- 
ity of 4.25 and grinds in 13 per cent of oil. 
As an inhibitive it stands fairly well, but, as 
noted, is not adapted to outside use unless 
mixed with other pigments. 

Willow charcoal is made by charring cer- 
tain kinds of wood, and contains a slight 
amount of alkali, which is probably the cause 
of its good inhibitive powers. It is a light 
pigment and grinds to a paste in 33 per cent 
of oil. 

Litharge, lead monoxide, is formed by heat- 
ing lead intensely for several hours. It is 
yellowish red, very heavy, and grinds in 9 
per cent of oil. It finds extensive use in the 
manufacture of boiled oils and is an excellent 
inhibitor. 



PAINT MATEEIALS 91 

Basic carbonate-white lead is made in sev- 
eral ways. The old Dutch process consists in 
placing lead plates or grids in clay pots with 
dilute acetic acid, stacking the pots up, and 
covering with tanbark. Fermentation of the 
latter causes a rise in temperature and the 
production of carbon dioxide, which acts on 
the lead in the pots and gives a basic lead 
carbonate. At the end of two months the 
white lead is ground in water and dried, be- 
ing ground later in oil. 

The ** quick process'' requires but two 
weeks and is carried out by acting on finely 
divided lead with dilute acetic acid and car- 
bon dioxide, from burning coke, in revolving 
cylinders. 

In the ''mild process" no acid is used, the 
finely divided lead being agitated in water 
through which air is blown. Hydrate of lead 
is formed and later carbonated. A very 
good product, free from all acid, results. 

The result of all of these methods is a very 
valuable pigment of specific gravity 6.S, 
grinding in 9 per cent of oil. It is very 
opaque and has much body, but is rather low 
in spreading power and is generally mixed 
with zinc oxide or pigments of similar na- 
ture. Sulphurous gases blacken it easily and 



92 CORROSION OF IRON 

it has a great tendency to chalk, due to the 
fact that it is naturally alkaline and thus acts 
on the linseed oil in the paint. It is generally 
inhibitive in its action and is widely used. 

Calcium sulphate, gypsum, is found in na- 
ture and is widely used in paint manufacture. 
It is very stable, but is somewhat soluble 
in water and has a tendency to wash out of 
paint films. Since it is easily ionized in the 
presence of water, with the formation of 
large numbers of hydrogen ions, it should 
not be used on iron or steel as corrosion is 
certain to occur. 

Prince's metallic brown is one of a number 
of iron ores which are used in paint manufac- 
ture. It is found as a natural hydrated iron 
oxide, or as a carbonate, and is prepared 
by roasting at a red heat and grinding. It 
contains a good deal of silica and alumina 
and is considered a standard pigment for 
protective paints. 

Orange mineral has the same composition 
as red lead, but a different tone, and is pro- 
duced by the oxidation of white lead. Owing 
to the variety of impurities which it contains 
its inhibitive power is variable. 

Calcium carbonate, whiting, is found exten- 
sively as chalk. When properly prepared it 



PAINT MATERIALS 93 

has a specific gravity of 2.8 and grinds in 
20 per cent of oil. It spreads well and is 
used to quite an extent, partly on account of 
its power of neutralizing any free acid in the 
linseed oil. The artificial form of this ma- 
terial is lighter and requires more oil for 
grinding. It generally contains impurities 
which make its use in protective paints in- 
advisable. 

Sublimed blue lead is made by burning a 
mixture of galena and soft coal. The fumes 
are drawn through cooling pipes and into 
large bags. The pigment has a specific grav- 
ity of 6.4 and grinds in 10 per cent of oil. It 
is bluish black in color and has given very 
good results when mixed with iron oxide or 
lampblack. 

Lemon chrome yellow is a mixture of sul- 
phate and chromate of lead and is made by 
precipitating a lead salt by bichromate of 
sodium and sulphuric acid. Owing to its 
occluded impurities it is not a good inhibitive. 

Orange chrome yellow is essentially a mix- 
ture of the neutral chromate and basic 
chromate of lead, and is made by precipitat- 
ing a lead salt by chromate of sodium and an 
alkali. It has a specific gravity of 6.9 and 
grinds in 20 per cent of oil. It contains a 



94i COEEOSION OP IRON" 

variety of impurities, but its inMbitive power 
is good if these are not acid in cbaracter. 

Medium chrome yellow is the pure neutral 
chromate of lead and is made from a lead salt 
and sodium chromate. Its specific gravity 
is 5.8 and it grinds in 30 per cent of oil. The 
strength of its color depends on the method 
of manufacture, and it is used as a tinting 
material. It is liable to carry down impuri- 
ties during the precipitation and therefore 
does not possess very reliable inhibitive pow- 
ers. 

Chrome green, oxide of chromium, is made 
from chromium salts. It is much used in rail- 
road work as it is very permanent and stable. 
In addition to other desirable qualities it is 
inhibitive to a slight extent, but its high cost 
prohibits its use in many cases. 

Venetian red is essentially ferric oxide and 
calcium sulphate, produced by heating iron 
sulphate and lime. It is not considered a 
good material to use on iron work on account 
of its content of calcium sulphate, which, 
as noted before, is fairly soluble in water 
and ionizes easily. Some varieties of this 
pigment are made by mixing oxides of iron 
with calcium carbonate and sulphate and, as 
they are free from acid, are safer for use. 



PAINT MATERIALS 95 

Bone black is made by heating bones to a 
high temperature for several hours and 
grinding the residue. It contains a large 
amount of calcium phosphate and carbon, has 
a specific gravity of 2.68, and grinds in 50 
per cent of oil. It is generally an excellent 
inhibitor and often replaces carbon black and 
lampblack in dark inhibitive paints. 

Asbestine is a natural silicate of magnesia, 
occurring also in the form known as talcose. 
Both varieties are very stable and are used 
in paints, largely to prevent the settling of 
other pigments and to strengthen the paint 
film. They are light pigments, grinding in 
32 per cent of oil. 

China clay is really aluminum silicate and 
is found in nature. It is a very permanent, 
fine, white powder and is much used in paints. 
It is a light pigment and grinds in 28 per cent 
of oil. 

Red lead is a bright red pigment made 
either by oxidizing litharge in furnaces or 
by heating it with sodium nitrate in iron pots. 
According to conditions of manufacture and 
other details the color varies somewhat. It 
is widely used for the protection of iron and 
is considered to be one of the best pigments 
known. It is generally mixed with oil, when 



96 COREOSION OF IRON" 

required for use, in the proportion of thirty 
pounds of pigment to one gallon of oil. It 
exerts such a drying action on the oil that no 
other drier is necessary. Sulphurous gases 
tend to turn it brown and it is often mixed 
with certain inert materials. Its inhibitive 
power varies with the method of manufacture 
and the impurities which it contains. 

Indian red is the name applied to certain 
hematite ores. They vary in shade and phys- 
ical characteristics, but are much used in in- 
hibitive and other paints. 

American vermilion is essentially a basic 
chromate of lead, being formed by boiling 
white lead and sodium chromate, with the 
addition of small amounts of sulphuric acid. 
It has a specific gravity of 6.8 and grinds 
in 16 per cent of oil. Its very good inhibitive 
properties are probably due to the presence 
of free chromates and would probably be 
increased by omitting the acid treatment. 

Sublimed white lead is a basic sulphate 
white lead made by volatilizing galena, lead 
sulphide. The fumes are oxidized in the air 
to a basic sulphate of lead and are drawn 
through cooling pipes to bags. It is very fine 
pigment of specific gravity 6.2 and grinds in 
10 per cent of oil. It is extremely stable in 



PAINT MATERIALS 97 

every way and is not blackened by sul- 
phurous gases, as are many pigments. 

Mineral black is made by grinding certain 
forms of slate and is mainly used as an inert 
filler for paints. What is called Keystone 
filler is made from a bituminous schist ore 
and contains a large amount of silica, with 
alumina and carbon. It is used widely on 
iron and steel, especially machinery. 

Barytes, barium sulphate, is found in large 
quantities in nature and is used extensively 
in paints. It is very stable and can be used 
as a base for the most delicate colors. It is 
very heavy and grinds in 10 per cent of oil. 
Blanc ^xe is the artificial form and is made 
by precipitating a barium salt by a solu- 
ble sulphate. In some ways it is a more 
valuable pigment than the natural form. 
Both varieties may contain acids and should 
be tested before application to iron sur- 
faces. 

Graphite occurs in two forms, the natural 
and the artificial. The latter is purer than 
the other, but both have about the same spe- 
cific gravity and grind in 45 per cent of oil. 
Both have an excessive spreading rate and 
are generally mixed with some of the lead 
or other heavy pigments. On account of 



98 CORROSION OF IRON 

their conducting power they are not con- 
sidered good inhibitors and should not be 
used as contact coats, although otherwise 
they form very valuable paints and are 
widely used. 

Ochre is similar to umber and sienna and' 
is essentially an iron-oxide pigment. These 
vary in their chemical composition and phys- 
ical characteristics and are not often used 
alone in paints for iron. 

Carbon black is made by burning natural 
gas and is very pure carbon. It is very light, 
grinds in 84 per cent of oil, and has been used 
in conjunction with white lead in protective 
paints, but it now appears to be a stimula- 
tor and as such is not safe to be used as a 
contact coat, although it can be employed 
very well as an excluder, on top of an in- 
hibitive paint. 

Lampblack is made by burning oils and is 
also a very pure form of carbon. It has a 
specific gravity of 1.82, grinds in 75 per 
cent of oil, and has unusual tinting power, 
wherefore it is used in large quantities for 
this purpose. Other characteristics are its 
great stability, its very slow rate of drying, 
and a preserving action on the oil with which 
it is combined. It resembles the graphites in 



PAINT MATERIALS 99 

its ability to conduct electricity; therefore 
it should not be used as a contact coat, but 
possesses perhaps unusual merit when used 
as a top coating. 



Chapter Y 

PEOTECTIVE PAINTS 

T7R0M the facts whicli have been presented 
-*- regarding the manufacture of paint mate- 
rials and the tests to which they have been 
subjected, it should be possible to predict, in 
a general way at least, the behavior of a pro- 
tective paint, provided we know its composi- 
tion. It must be noted, however, that there 
are many things to be considered in designing 
or passing on the probable value of a paint 
from its formula or analysis, as the question 
is not altogether whether a good quality of 
linseed oil and stimulative or inhibitive pig- 
ments have been used, although these points 
are of great importance. 

In the first place, it appears that even 
though the inhibitive powers of the pigment 
are the main factor in protecting a painted 
surface from rust, this action is enhanced 
greatly by the use of pigments which are 
good insulators, as far as the ability to con- 
duct a current of electricity is concerned, for 

100 



PROTECTIVE PAINTS 101 

the base. Again, a study of paint films has 
shown that some are much better excluders 
or water shedders than others. It has been 
shown that linseed-oil films are markedly 
porous, with the result that water soon pene- 
trates to the surface below and, if it is of iron, 
sets up corrosion. Most dried paint films are 
porous, although this porosity may be les- 
sened or prevented by the use of varnish, 
thus producing a film which is a good ex- 
cluder. By the use of finely ground pigments 
the size and number of pores in a film may be 
very materially reduced, but even then the 
film is not necessarily a water shedder. Cer- 
tain pigments seem to convey a peculiar prop- 
erty to a film in that it becomes wet with 
difficulty or not at all. Water rapidly evapo- 
rates or runs off from it and apparently none 
is retained upon the surface or in the pores. 
Perhaps the action of different pigments in 
excluding water from paint films may best be 
shown by citing an experiment performed 
and described by Cushman and Gardner. 
A number of pigments were ground in oil 
and films made therefrom. It was not pos- 
sible to make all films of exactly the same 
thickness, but there is no doubt that these 
results are roughly indicative of the ability 



102 



CORROSION OF IRON" 






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104 CORROSION OF IRON" 

of these materials to act as excluders. The 
method of making the test was as follows : 

A series of small glass bottles with wide mouths, 
holding about two ounces, were half filled with con- 
centrated sulphuric acid, and paint films were tightly 
sealed over the mouths of the bottles, using Canada 
balsam. The bottles were then carefully labeled, 
numbered, accurately weighed on chemical balances, 
finally exposed to air saturated with moisture and 
kept at constant temperature under a large glass re- 
ceptacle. The bottles were removed from the re- 
ceptacle every week and weighed. The increase in 
weight, due to the amount of moisture which had 
penetrated the films, and which had been taken up 
by the sulphuric acid, owing to its hygroscopic na- 
ture, was thus determined. 

It is interesting to note the effect which a 
varnish gum has in closing the pores of a 
vehicle, as shown in this test. Thus iron 
oxide used alone stands about half way down 
the list, but when it contained 2 per cent of 
gum in addition to the oil it stood very high. 
It has been shown that the addition of a high- 
grade Kauri gum varnish to a paint increases 
its protective power considerably, this in- 
crease, up to a certain point, corresponding 
to the amount of varnish added. 

Putting together all of the above facts and 
coming to the practical design and testing of 
protective paints, The American Society for 



PROTECTIVE PAINTS 105 

Testing Materials has for a long time been 
carrying on very thorough studies along these 
lines. The results of these tests, which have 
been made under service conditions as far 
as possible, are available in the reports of 
the Society, but for the sake of the readers 
who do not have access to them or who do 
not have time to look them up and go through 
the many papers, they will be given here in 
part, as the writer feels they will be of more 
value than any theoretical discussions could 
be. 

One of the first tests was on the Pennsyl- 
vania Eailroad bridge over the Susquehanna 
Eiver at Havre de Grace, Maryland. Nine- 
teen different paints were applied to panels 
placed on the bridge and to the bridge itself, 
brushing coats of definite weights. Very 
careful data were kept of all the conditions 
of the test, analyses were made of the paints, 
and everything, including the inspections, 
was carried out with a great deal of thor- 
oughness. The final results are now avail- 
able, but it is considered inadvisable to con- 
sider them further here, and mention is made 
only because it is thought that some reader 
of this volume might wish to make a thor- 
ough study of such a test. 



106 



CORROSION OF IRON 



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PROTECTIVE PAINTS 



107 



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COMPOSITION OF SPECIAL FORMULAS 

Pigment Form\ila Ingredients Paint Formula 

No. Ill 60 Burnt Umber 22.50 

20 Zinc and Barium Chromates ... 7 . 50 

20 Zinc Lead 7.50 

Japan 3 . 40 

Raw Oil 59 . 10 

No. 333 35 Zinc Oxide 20.90 

45 Zinc Lead 26.87 

5 Calcium Carbonate 2 . 98 

15 SHex 8.95 

Japan 1 . 56 

Raw on 38.74 

No. 555 40 Lampblack 8.18 

40 Natural Graphite 8.18 

20 Barytes 4 .09 

Japan 8 . 33 

Raw Oil 71.22 

No. 777 60 SubHmed Wliite Lead 44 . 80 

20 Blanc Fixe 7.46 

20 GjTsum 7.46 

Japan 1 . 60 

Raw Oil 38.68 

No. 222 30 Bone Black 13.72 

2 Prussian Blue 0.92 

10 XX Zinc 4.57 

50 SHex 22.86 

8 Calcium Carbonate 3 . 66 

Japan 8 . 33 

Raw on 45.94 

No. 444 60 XX Zinc 32.47 

15 Zinc Chromate 8 . 30 

3 Prussian Blue 1 1.44 

2 Calcium Carbonate 1 .08 

20 Snex 10.83 

Japan 2.22 

RawOn 43.66 

No. 666 50 Red Oxide, 62a 22.94 

5 Carbon Black 2.35 

35 Barytes 15.30 

10 Med. Chrome YeUow 5 .88 

Japan 3 . 20 

Raw on 50.33 

No. 888 5 Chinese Blue, S 4.32 

35 Lemon Chrome Yel 18 .94 

20 SubUmed White Lead 16 . 23 

40 Barytes 21.35 

Japan 2 . 35 

RawOn 38.51 

108 



PROTECTIVE PAINTS 109 

Some time after this test was started the 
Paint Manufacturers Association of the 
United States offered to erect and put under 
observation of Committee *^A-5" of The 
American Society for Testing Materials a 
series of steel panels to which had been ap- 
plied paints made up with a variety of pig- 
ments, used alone and in conjunction with 
others, and work was started in the autumn 
of 1908. The table on pages 106 to 108 is 
taken from the report of Committee A-5, 
Proceedings of The American Society for 
Testing Materials, Vol. 10, 1910. 

All the pigments were ground in mixture 
of two parts raw and one part boiled linseed 
oil, the weight of pigment per gallon of oil 
being indicated in the table. In preparing 
the paints for test it was so arranged that 
the same amount by volume of each pigment 
would be used in the same quantity of oil, 
making use of the formula. Specific gravity 
of pigment X 3 == pounds pigment per gal- 
lon of oil. In order to avoid the introduction 
of unknown factors, japan driers were not 
used. 

Several hundred steel plates 24 by 36 in- 
ches were obtained and one-half of them 
pickled in sulphuric acid, neutralized in alkali 



110 CORROSION OF IRON 

solution, washed, and preserved from rusting 
until needed. The painting of the plates was 
doiie very carefully under cover, a spreading 
rate of 900 square feet to the gallon being 
used on the pickled plates ; on the unpickled 
samples no definite spreading rate was used. 
The coats were allowed sufficient time to dry 
thoroughly ; three coats were given regularly 
to the pickled plates upon which the original 
fifty-one pigments were applied, but in the 
case of the special formulas only two coats 
were applied to the black plates. 

In a few cases the viscosity of the paste 
was so great that it was necessary to add a 
small amount of pure turpentine, but in these 
the amount of paint applied was increased 
proportionately so that the weight originally 
intended was left when the solvent evapo- 
rated. In all other cases the paints were 
used just as they were made. 

The painted panels were placed on three 
fences near Atlantic City, N. J., each of which 
is 125 feet long and 5 feet high. These were 
strongly built of yellow pine and carefully 
designed to serve the purpose for which they 
were intended. 

Several inspections of the panels have been 
made, the last available results being ob- 



PROTECTIVE PAINTS 111 

tained in April, 1913. The average of the 
ratings given to each sample by the different 
inspectors on the second, third, and fourth 
annual examinations is given on pages 112 
to 114. 

The ratings assigned to each panel were 
based on the amount of rust apparent on the 
painted surface, as well as the degree of 
checking, chalking, scaling, peeling, cracking, 
loss of color, and other signs of paint failure 
shown. The numeral 10 represents the best 
condition and the paints are given various 
markings down to zero, which represents fail- 
ure. 

This table is condensed from tables given 
in the Proceedings of The American Society 
for Testing Materials. 

Shortly after the last inspection it became 
necessary to remove the test fences, in order 
to clear the land for building purposes, and 
those samples which had failed were dropped 
from further consideration. They included 
the following: Nos. 1, 2, 3, 6, 7, 27, 28, 29, 30, 
31, 32, 33, 45, 48, 222, 333, 777, 2000, 3000, 
4000, 90, 100, 5555. 

These results are largely self-explanatory 
and a detailed discussion of them would 
hardly be possible or necessary at this time, 



112 



CORROSION- OF IRON" 






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PROTECTIVE PAINTS 115 

but a few comments may not be out of place. 
First, it will be noticed that not all of the 
pigments have shown up as might have been 
expected from the results of the water sus- 
pension test, quoted in the preceding chapter. 
In other words, some of the materials which 
were considered to be inhibitive in their ac- 
tion showed up considerably inferior to some 
which were thought to be stimulators. There 
are probably several reasons why such a re- 
sult was obtained. It has been previously 
noted that a pigment in water suspension is 
under vastly different conditions from one in 
a paint film, that is, in oil. It will be recalled 
that in the preparation of the paints the 
same amount by volume of each pigment was 
ground in the unit quantity of oil. Pigments 
vary greatly in the amount of oil which is 
required to grind them into paste or paint 
form, and there can be little doubt that dif- 
ferent results would be obtained if various 
quantities of oil had been mixed with each 
pigment. 

Again, the fineness of the pigment makes a 
great deal of difference to the paint, as it 
determines the extent to which the pores and 
voids in the oil film will be filled up, thus 
producing a more impervious film. Then, too. 



116 CORROSION OF IRON 

the freedom of the pigment from impurities 
can easily be seen to be of prime importance. 
In this test the materials were of commercial 
purity. 

In the case of paints made from a combina- 
tion of pigments, the deviation of the results 
obtained from those which might have been 
expected was considerable. In such a case 
we have to regard the action of several pig- 
ments in a film, instead of one, and at present 
such action is not always easily foretold. 
There is no doubt that the proper combina- 
tion of two or more pigments makes a better 
and more durable paint than one used alone. 
In the main this effect is mechanical or phys- 
ical, each component contributing certain 
properties toward the production of a film 
having the desired qualities. 

On the whole, it seems that these tests 
show what each of the pigments tested has 
done, and probably would do regularly, when 
made up in a certain vehicle and in a cer- 
tain way, and that valuable data have been 
obtained for use both in the practical design- 
ing of protective paints and in the carrying 
out of future experimental work. 

In working up a paint formula attention 
must be paid to the particular requirements 



PROTECTIVE PAINTS 117 

of each case, since no one combination will 
give good results in all situations. For in- 
stance, a paint which must stand exposure 
to light and heat is of different composition 
from one which would be used in a cool, damp 
place. It is not possible to consider this ques- 
tion further, but it is hoped that the data 
which have been given will be of at least 
some assistance along these lines. 

The method of protecting iron, particularly 
building and structural material, employed 
by the manufacturers of ^^ Asbestos Protected 
Metal, ' * is worthy of notice. Briefly, the proc- 
ess consists in cleaning and heating the parts, 
then immersing them in some asphaltic com- 
pound, also kept at a high temperature. Upon 
removal they are passed through rolls, to se- 
cure more perfect union of the steel and the 
coating compound and remove air bubbles, 
then through another set of rolls between two 
sheets of asbestos felt, made from a special, 
long-fibred material. 

Of course, the use of asphaltum in one way 
or another as a protective agent is not new 
and when properly prepared it may give very 
good results. Heating the iron before the ap- 
plication of the coating is an excellent plan, 
as all moisture is thereby removed from the 



118 COEEOSIOX OF lEON" 

surface and pores; the advantage in doing 
this is easily seen. The asbestos acts as a 
mechanical protection to keep the nnderlying 
coating from injury by blows or scratches 
and shields it from the destructive action of 
the sun and air. The final stage of the proc- 
ess is a treatment designed to make the as- 
bestos water-proof, so it would appear that 
parts protected in this manner should last 
almost indefinitely. 

Coming now to protective materials which 
are slightly different from the foregoing, so- 
called baking japans are used widely in fin- 
ishing a large variety of metal work requir- 
ing a decorative finish which is cheap and 
durable. In general, black japans are made 
by combining linseed oil and asphalt, gilson- 
ite, stearine pitch, coal-tar pitch, and similar 
substances at high temperatures. The re- 
sulting product is thinned with turpentine, 
benzine, or light mineral oils, and when ap- 
plied by spraying or dipping and baked at a 
temperature ranging from 200 degrees F. to 
450 degrees F. or so, will give a more or less 
thick coating which is quite hard, fairly im- 
pervious, and of a high lustre. The baking 
period, which varies from two to six hours or 
longer, and the temperature of baking, de- 



PROTECTIVE PAINTS 119 

pend on the composition of the japan. Dull 
and medium gloss japans are made by grind- 
ing the bright japan with lampblack or car- 
bon black in a fine mill. The method of ap- 
plication and baking is the same as given. 

Colored japans or enamels are essentially 
pigments ground in a boiled linseed oil which 
may or may not contain a percentage of var- 
nish gums. They are generally baked at con- 
siderably lower temperatures than the black 
japans. 

The film given by these baking japans is 
much harder than an ordinary paint film and 
will stand rougher handling and usage, a cir- 
cumstance which gives them a marked advan- 
tage in many cases. While they are hardly 
intended for outdoor exposure and perhaps 
are not as durable and resistant under such 
conditions as the best oil paints, they give, 
nevertheless, very good protection even un- 
der severe service. Two or three coats may 
be applied in much less time than it takes for 
one coat of paint to become thoroughly dry, 
so this is an added advantage of some im- 
portance. Altogether, baking japans furnish 
a means of producing quickly and cheaply 
a coating which has good protective and 
decorative qualities. 



120 COEKOSION OF IRON^ 

An interesting application of this sort of 
process is in the coating of large pipe for 
water mains and so on. Many years ago Dr. 
Eobert Angus Smith, an English scientist, 
patented a method of treating such objects 
by dipping them in a mixture of coal tar, 
from which most of the volatile constituents 
had been distilled, and linseed oil, kept at 
a high temperature. The part was thor- 
oughly cleaned and coated with linseed oil, 
which was baked on, then it was coated with 
the coal-tar mixture and the residual heat 
of the pipe served to bake this on. In pass- 
ing, it is worthy of comment that not very 
much improvement has been made in the 
matter of coating cast-iron pipes since the 
time of Dr. Smith. The application of a 
coal-tar coating does not give very good re- 
sults on sheet-steel riveted pipe, however, and 
a more or less soft asphaltum is now used. 
A material known as maltha and obtained 
from California petroleum as a residue which 
remains in the retort after the more volatile 
portions have distilled off, is used to a large 
extent for this purpose. The dipping ma- 
terial is thinned with high boiling mineral 
oils and is heated to a high temperature for 
application, but the pipe is not baked after- 



PROTECTIVE PAINTS 121 

ward, the coating being allowed to cool in 
the air. 

Sabin has improved this process by the use 
of asphaltum and linseed oil, whereby it is 
claimed to be possible to obtain a coating of 
any desired degree of elasticity by adding the 
requisite amount of the latter and baking 
properly. The use of volatile solvents is 
avoided by heating the compound to 300 de- 
grees F. for application and a thin uniform 
coating results. After dipping the pipe is 
allowed to drain for a time, until the excess 
has run off, which ordinarily takes one half- 
hour or less. It is then put in the oven, which 
is heated somewhat above 300 degrees F., 
and allowed to remain two hours or so, or 
until the volatile substances have been driven 
off and the oil oxidized. A tough, hard coat- 
ing is produced and the Sabin process, as 
it is called, is finding a wide use. It is inter- 
esting to note that the steel underfloors of the 
roadways and sidewalks of the Williamsburg 
bridge over the East Eiver in New York City 
were pickled and coated in this manner, with 
very satisfactory results. 

A coating for steel and wrought-iron pipe, 
thoroughly protected by some sort of heavy 
fabric, is much superior to one produced by 



133 CORROSION OF IRON 

simple immersion in a bath of the hot bitum- 
inous compound, and the ** National Coat- 
ing," produced by the manufacturers of Na- 
tional pipe, may be mentioned as a sample of 
this type. 

In applying this the pipe is cleaned and 
dried, then dipped at a temperature above 
the boiling point of -^ater into a hot bath 
of a special refined bituminous compound, 
after which it is removed and allowed to 
drain in a vertical position and cool. The 
next operation consists in placing it in a 
winding machine where a strip of fabric, 
saturated in the hot compound, is wound 
spirally over the surface so as to overlap an 
inch or so on each turn; if necessary two or 
three layers of fabric may be applied. The 
final result is a hard, but tough and elastic, 
coating of considerable thickness which is 
strengthened and protected by the heavy 
fabric embedded in it. 

Naturally, gas and water pipes are liable to 
undergo rather strenuous treatment before 
they are finally laid to rest in the ground, and 
an ordinary coating may be badly scraped 
and damaged, thus greatly reducing its use- 
fulness. A coating such as described above, 
however, is very resistant to mechanical in- 



PROTECTIVE PAINTS 123 

jury and should remain in good condition al- 
most indefinitely. 

There is still another class of protective 
materials, known as lacquers, which may be 
considered briefly here, although they afford 
relatively very slight protection and are not 
used to any great extent on iron. In general 
there are three kinds of lacquers — gum or 
resin, pyroxylin, and those made by combin- 
ing these two. 

The first kind is made by dissolving shellac, 
kauri, damar, sandarac, and so on, in alcohol, 
fusel oil, or other solvents. Eesin lacquers 
are applied by brushing, as a rule. 

A pyroxylin lacquer is essentially a solu- 
tion of cellulose nitrate in amyl acetate. The 
nitrate is generally made by treating cotton 
with a mixture of nitric and sulphuric acids, 
whereby a reaction takes place in which the 
cellulose unites with two or three molecules 
of a nitrogen compound, depending on the 
strength of the acid and the temperature and 
time of treatment. The cotton is then washed 
and all acid neutralized, and after drying is 
dissolved in a suitable solvent. Amyl acetate 
is perhaps the best, but it is very expensive, 
so is often adulterated with wood alcohol, 
acetone, benzine, or other substances. 



124 COREOSION OF IRON 

The finest lacquers of this class are made 
from nitrated tissue paper, since this gives 
a whiter and better appearing product than 
cotton. This '* water white'' lacquer is used 
almost exclusively on silverware. 

The combination lacquers are made by mix- 
ing solutions of pyroxylin and resins, thereby 
producing a material which has a far wider 
range of usefulness than either of the pre- 
ceding. By the proper blending and mixing 
the most desirable qualities of both of the 
constituents may be brought out — ^hardness 
and covering power may be increased with- 
out increasing the flow, and so on. Probably 
the combinations of pyroxylin and shellac are 
the best and most widely used. 

Any of the above lacquers may be colored 
by grinding pigments in them or by adding 
suitable dyes. They are mostly used, of 
course, for decorative work. All of these ma- 
terials are applied by dipping, brushing, or 
spraying, depending on the nature of the 
work and the qualities of the lacquer, and are 
allowed to dry in the air or subjected to 
gentle heat to hasten the process and assist 
in the production of a harder coating. As 
noted before, lacquers have a rather limited 
field of usefulness on iron work, but they are 



PROTECTIVE PAINTS 125 

used very extensively on brass and copper 
articles, fixtures and the like. 

In conclusion, there is one important item 
which has been mentioned only incidentally 
before, but which is deserving of thorough 
treatment and consideration, and that is the 
proper cleaning of the surfaces of iron and 
steel parts before paint or other protective 
materials are applied. Paint which is put 
on a surface covered with grease or dirt does 
not come into good contact with the metal 
and consequently does not adhere to it. 
Sooner or later the dirt will fall or be 
knocked off and the paint will come with it. 
Beams and other parts which are coated with 
heavy oil or which have been allowed to lie 
on the ground until they are covered with 
mud and dirt, should be thoroughly cleaned 
before painting if it is desired that protection 
of any permanence shall be secured. Mill- 
scale is another substance which is almost 
always found on iron work, especially struc- 
tural material. It is hard and perhaps has 
some protective power when firmly attached, 
but all loose scale should be removed as com- 
pletely as possible by hammer and chisel or 
sand-blasting, as it tends to crack off slowly, 
taking the paint with it and exposing the bare 



136 COREOSION OF IRON" 

iron. It is a matter of common experience 
that when rust once gets started it is very 
difficult to stop it, so progressive rusting may 
take place under a paint film which is itself 
intact and in good condition. It is important, 
therefore, to see that all surfaces are free 
from active rust, loose mill-scale, dirt, and 
other foreign substances, and that they are 
perfectly dry before painting. In addition, 
warming the parts slightly, whenever this is 
possible, just before the paint is applied, also 
tends to promote better adherence and more 
satisfactory results all around. It must be 
remembered, too, that these remarks apply 
not only to the first, but to all subsequent 
paintings. 



Chapter YI 

INFLIJENCE OF DIFFEEENT ELEMENTS ON 
THE COEEOSION OF lEON 

T T has probably occurred to everyone who 
"■- is forced to rely on protective coatings of 
one sort or another to keep iron and steel 
from rust and decay, that this method of com- 
bating the corrosion problem is of the same 
order as locking the barn door after the horse 
is stolen. In other words, since rusting is 
a consequence of the tendency of iron to go 
into solution when in contact with water, why 
not add something to it or treat it in some 
way so as to eliminate or decrease this ten- 
dency? An enormous amount of labor and 
material, to say nothing of time, is used up 
every year in coating and protecting iron 
from adverse conditions, and with the rapid 
increase in the demand for this metal a cor- 
respondingly greater amount of labor, ma- 
terial, and time is required. Obviously, any 
procedure which would lessen the need for 
such thorough protection would be a very 
real saving. 

127 



128 CORROSION" OF IRON 

Definite work along this line is of com- 
paratively recent date, and the available in- 
formation is perhaps not as complete as 
could be wished; but enough has been done 
to throw some light on the possibilities of 
ultimate success and to enable us to use to 
good advantage some of the facts which have 
been obtained. 

In effect, of course, the problem resolves 
itself into the study of the influence of the 
various elements (meaning the different 
metals and metalloids) on the process of cor- 
rosion. Chemically pure iron is little more 
than a laboratory curiosity; certainly it is 
not available for commercial use in building 
bridges and sky-scrapers, and experiments 
have shown that, on the whole, it is very little 
or no more resistant to corrosion than well 
made wrought iron or steel ; so the answer to 
the problem does not lie in this direction, al- 
though this should not be taken to mean that 
any pains are to be spared in making iron and 
steel of high purity and as free from segrega- 
tion as possible. Perhaps a better idea of 
the difficulties presented in accomplishing 
these ends may be obtained by reviewing 
briefly the early stages of the manufacture 
of iron. 



ELEMENTS INFLUENCING CORROSION 129 

This metal is found in nature principally 
in the form of the oxide, and by far the larg- 
est part of the ore is smelted in blast fur- 
naces, which are a sort of immense stove, 75 
or 100 feet in height. The ore, mixed with 
coke (more rarely with charcoal or coke and 
anthracite coal) and enough flux (generally 
limestone) to remove some of the impurities 
in the form of slag or cinder, is charged in at 
the top. Air heated to a temperature of sev- 
eral hundred degrees Centigrade is forced at 
a pressure of about 15 pounds per square inch 
into the furnace near the bottom, through 
pipes known as tuyeres. This blast is further 
heated by the combustion of the fuel and 
passes up through the charge, melting and 
reducing the ore. The metallic iron formed 
trickles down and collects on the bottom of 
the hearth, with the cinder floating on top of 
it; the furnace is tapped at intervals of sev- 
eral hours, and the iron cast into pigs or sent 
to the steel mill where it is purified as will 
be described in the next chapter. The cinder 
is allowed to escape at longer intervals. 

The iron produced in this manner is a com- 
paratively impure material, since it contains 
very considerable amounts of carbon and sili- 
con, especially, and less of manganese, sul- 



130 CORROSION OF IRON" 

phur and phosphorus, with varying quantities 
of several other elements, depending on the 
character and quality of the ore. Altogether, 
these impurities will amount to 6 or 7 per 
cent. Pig iron is melted and cast into various 
forms, constituting the gray or cast iron of 
the foundry, but it can not be rolled or other- 
wise worked without further treatment. 
Omitting for the present the details of this 
subsequent treatment, the product is known 
as wrought iron or steel, which latter is a 
rather indefinite term. In these, however, 
the percentage of other elements will gener- 
ally be well under 1 per cent, except where 
one or more of them is added purposely, as 
in making alloys, or for conferring certain 
desirable characteristics. It is impossible to 
remove all of the sulphur, phosphorus, and 
so on, and if the attempt is made to do so, 
the iron itself undergoes considerable oxida- 
tion, the oxide present mingling with the mol- 
ten metal and introducing a new and perhaps 
greater danger. 

All the ordinary impurities in iron, with 
the exception of manganese, are electro-nega- 
tive to it, which means that a current will flow 
from one to the other in the presence of an 
electrolyte. Under such circumstances the 



ELEMENTS INFLUENCIN-G CORROSION" 131 

solution of the iron will be hastened, as ex- 
plained in a preceding chapter, and it will 
corrode much more quickly. It is in a way- 
reasonable, therefore, for the idea to have 
arisen that the purer steel and iron can be 
made, the less subject to corrosion will they 
be, thus rather tacitly blaming the tendency 
to rust on the inherent impurities of the iron. 
Without stopping to discuss the soundness of 
this view, the fact remains that it is not prac- 
ticable to refine iron beyond a certain point ; 
hence experiments have been tried to ascer- 
tain the effect of adding other elements be- 
sides those usually found in iron, in the hope 
that in some way the end desired might be 
accomplished by this means. 

Taking up first the normal constituents of 
iron and steel, it appears from the data at 
hand that silicon rather tends to increase cor- 
rosion, although this is contrary to what 
might be expected as it is well known that 
high silicon irons are markedly resistant to 
acid attack. It is generally agreed, however, 
that the resistance to acid is not always a re- 
liable measure of corrodibility, since many 
discrepancies are observed and perhaps this 
is one of them. The silicon content of ordi- 
nary steel is not very high, so this element, 



132 CORKOSION OF IRON 

when present in normal amounts, probably 
exerts little effect as far as corrosion is con- 
cerned, but even if it did have an inhibitive 
action it could hardly be used in any quantity 
as it renders steel weak and brittle. 

Sulphur is harmful in its effects, materially 
increasing the tendency to corrode; even 
fairly small amounts exert an appreciable in- 
fluence in this direction. 

Manganese is popularly supposed to be the 
cause of much trouble, and the supposedly 
greater resistance of the older irons over 
modern steel has been largely ascribed to 
their low content of this metal. Recent work, 
however, shows that it does not play as im- 
portant a part as has been thought. In the 
tests conducted by Burgess and Aston the 
results were not altogether consistent, but 
in general it appeared that atmospheric cor- 
rosion increased with higher manganese con- 
tent, although in all cases this was less than 
that observed for pure electrolytic iron. On 
the whole, probably the influence of manga- 
nese, unless it is present in amounts greatly 
in excess of what would ordinarily be the 
case, may be considered as practically negli- 
gible; at any rate its action is not strongly 
marked one way or the other. 



EliEMENTS INFLUENCING CORROSION 133 

Phospliorus seems to exert a beneficial ef- 
fect, rather than otherwise, when present in 
reasonable quantities. Of course, the phos- 
phorus content of a steel affects the physical 
properties greatly and hence the permissible 
amount is low ; specifications are usually very 
strict on this point. 

Carbon is another element which deter- 
mines to a large extent the characteristics of 
iron and steel, and the amount allowable is 
rather limited ; but from some investigations 
it appears that the corrodibility of annealed 
steel rises with carbon content up to about 
.89 per cent., then decreases. In quenched 
and tempered steels no decrease in corrodi- 
bility could be noted up to .96 per cent, car- 
bon. Aside from this the effect of varying 
amounts of some of the other normal impuri- 
ties of iron has apparently not been studied 
in detail, but it is rather doubtful if results 
of any particular value would be obtained. 
These metals and metalloids are always pres- 
ent to some extent in iron and steel; to re- 
move them entirely is a very difficult if not 
impracticable thing commercially, and in any 
case the weight of evidence goes to show that 
pure iron corrodes, in the long run, at a rate 
little or no different from an ordinary, com- 



134 CORROSION" OF IRON" 

mercial grade of material, assuming that this 
is well made and reasonably free from segre- 
gation. The point is, that none of these ele- 
ments, with the possible exception of sulphur, 
appears to exert any particular influence, 
either good or bad, on the process of corro- 
sion when they are present in the amounts 
permissible from the standpoint of the physi- 
cal properties which they confer. There 
seems to be no reason to hope, therefore, that 
any benefits will accrue either from removing 
them completely or increasing the amount of 
any or all of them over that usually met with. 

Some time ago Burgess and Aston carried 
out experiments on the alloys produced by 
adding various quantities of different metals 
to electrolytic iron — that is, iron which had 
been deposited from a solution of a salt of 
this metal by means of the electric current 
and therefore was of great purity. Care was 
taken to exclude impurities as far as possible, 
in making up the samples, and after the al- 
loys had been brought into proper shape for 
testing, small pieces were exposed to the 
weather for something less than six months, 
then cleaned and the loss in weight noted, and 
an accelerated acid corrosion test applied. 

Before considering the results it may be 



ELEMENTS INFLUENCING CORROSION 135 

noted that these tests were not made on a 
commercial grade of iron, the electrolytic ma- 
terial being practically free from carbon, sili- 
con, manganese, sulphur and phosphorus and 
other impurities so that the effect of these 
was lost, but while this may be objectionable 
in some ways there is little doubt that the 
results indicate, in a general way at least, 
the influence of the metals tested. 

In trying out various grades of commercial 
irons of different degrees of purity, it was 
found that some corroded more, some less, 
than the electrolytic, the samples of the latter 
representing an average product, as far as 
composition is concerned. In its ability to 
withstand acid attack or atmospheric corro- 
sion it did not stand out at all prominently, 
and these results are in general accord with 
those obtained by other investigators; that 
is, on very pure iron rust may be somewhat 
slower in starting than with the ordinary va- 
rieties, but the difference in behavior soon be- 
comes negligible and in the course of an ex- 
posure test covering several months it ap- 
pears that there is no decided superiority one 
way or the other. 

A sample of pure iron containing 1.33 per 
cent of aluminum showed a slightly decreased 



136 CORROSION' OF IRON 

tendency to corrode in the air, but was more 
easily attacked by acid. Neither it nor an- 
other sample containing a much smaller 
amount of aluminum gave results of any par- 
ticular interest. 

Arsenic confers no especial resisting power 
against atmospheric corrosion, even when 
used in amounts up to 3.56 per cent. It is 
well known that this metal is an objection- 
able impurity in sulphuric-acid pickling solu- 
tions as it slows down the action consider- 
ably, and it has been thought that something 
of this sort might be exhibited when it is al- 
loyed with iron, but apparently the reverse 
is true since the rate of solution in acid be- 
comes higher with larger amounts of it pres- 
ent. 

In any case iron-arsenic alloys possess un- 
desirable qualities, such as weakness and brit- 
tleness, which would render them unsuited 
for structural purposes, aside from the fact 
that they are rather difficult to make. 

In like manner, the alloys with cobalt do 
not present any unusual characteristics. The 
resistance to acid attack increases with higher 
cobalt content, and this is true of the behav- 
ior of the specimens exposed to the weather. 
As in the case of arsenic, considerable per- 



ELEMENTS INFLUENCING CORROSION 137 

centages of cobalt would be required to pro- 
duce an alloy having sufficient resistance to 
corrosion to be of much value. The com- 
paratively high cost of the metal is one ob- 
jection to the use of such alloys and, further, 
the addition of such large quantities of it 
changes the properties of iron and steel more 
or less profoundly and takes us into the realm 
of special steels. This is also true of several 
other metals besides cobalt. 

The addition of nickel gives results some- 
what similar to those obtained with cobalt; 
perhaps this might be expected in view of the 
close chemical relationship between the two 
metals. Nickel, however, confers very fair 
resistance to both acid and atmospheric at- 
tack and this corresponds in a general way 
to the amount of this metal present. 

As in previous cases, however, a fairly 
large amount of nickel is needed (1 per cent 
or over) to produce very decided results from 
the standpoint of corrosion, and this in- 
creases the cost while altering the physical 
properties as well; up to about 3.5 per cent 
it gives greater strength, raises the elastic 
limit and promotes soundness. Iron-nickel 
alloys have properties which have been found 
of value for certain purposes, but it is rather 



138 CORROSION" OF IRON" 

doubtful if they come into wide use for gen- 
eral work solely on the score of decreased 
corrodibility. 

Experiments with silver, tin, tungsten and 
selenium indicate that these elements when 
alloyed with iron do not alter very much its 
tendency to corrode. In the case of silver 
and selenium the alloys contained only small 
amounts of these metals and it would be un- 
wise to attempt to draw very definite con- 
clusions as to their action. In any event it 
is not likely that either would be used very 
widely, on account of the cost. 

With both tin and tungsten there is a tend- 
ency toward increased resistance to atmos- 
pheric corrosion as the content of these met- 
als in their respective alloys becomes greater, 
but it is nothing remarkable at any time ; the 
alloys with low tin content display very good 
resistance to acid attack, but this decreases 
appreciably with the higher alloys. The cor- 
responding figures for tungsten show up well, 
but they are irregular and lack consistency. 
As in the other instances, these metals, to be 
of much effect, would have to be added in 
amounts which are sufficient to change the 
properties of the iron markedly; in general 
the iron-tin alloys are brittle and lack 



ELEMENTS INFLUENCING CORROSION 139 

strength, while those with tungsten are noted 
for their great hardness and are used under 
the name of high-speed tool steels. Obviously 
they would be unfit for ordinary purposes, 
aside from cost considerations. 

Copper may be regarded as a normal con- 
stituent of iron and steel, although it is some- 
times present only in the barest traces, but 
the effects which even small additions of it 
produce are remarkable and a considerable 
amount of very careful work has been done 
in investigating the possibilities which it pre- 
sents along the line of reducing the tendency 
of iron to corrode at the least opportunity. 
The results are so interesting that they are 
worth considering in some detail. 

As long ago as 1901 two experimenters. 
Stead and Wigham, made some tests on cop- 
per- and non-copper-bearing steels and came 
to the conclusion that, under the conditions 
of their tests, at least, the steels with copper 
showed up to better advantage. Within the 
last few years, however, other experimenters 
have taken up this subject, and of these Buck 
has probably done more than anyone else 
and his work merits more than passing atten- 
tion. 

In a paper read before the annual meeting 



140 CORKOSIOI!^ OF IKON" 

of the American Chemical Society at Mil- 
waukee, March 25, 1913, he gave the follow- 
ing information regarding the preparation of 
the samples and the method of conducting the 

tests. 

In order to avoid the possible uncertainty in com- 
paring different heats of steel with and without cop- 
per, and in order that the conditions, except the cop- 
per content, should be identical, it was decided for 
these comparisons to copperize portions of heats, leav- 
ing other portions of the same heats in their original 
conditions. 

Three heats were used. One was a regular basic 
open-hearth of the following analysis: 

Carbon Manganese Sulphur Phosphorus 



.10 .34 .034 .019 

A second basic open-hearth heat was re-phosphor- 
ized, giving this analysis : 

Carbon Manganese Sulphur Phosphorus 



.13 .45 .036 .042 

The third heat was regular Bessemer steel of the 
following analysis: 

Carbon Manganese Sulphur Phosphorus 



.08 .46 .070 .096 

In pouring the open-hearth heats several ingots 
were first poured without the introduction of copper, 
then to four ingots sufficient copper was added to ob- 
tain in two of them about .15 per cent, and the 



ELEMENTS INFLUENCING CORROSION 141 

other two about .25 per cent copper in the finished 
product. The Bessemer heat was treated in exactly 
the same way, except that, since the average Bessemer 
heat is too small to furnish six ingots of the size 
desired, only two ingots were copperized, aiming at 
the same contents as in the case of the open-hearth. 
The copper was added to the moulds a little at a 
time as they were filling, and that the resultant steel 
was uniform in its copper content, was demonstrated 
by many analyses of the bars and of the finished 
sheets. Indeed, that copper easily diffuses through 
the bath of molten steel, and does not segregate on 
cooling, is a well established fact. 

Six ingots were then taken from each of the open- 
hearth heats, two plain, two with .15 per cent copper 
and two with .25 per cent copper, and three ingots 
from the Bessemer heat, one plain, and one with each 
content of copper. 

The fifteen ingots thus prepared were carried 
through the usual mill operations, each bar as cut 
and each sheet as rolled being chalk-marked so that 
no confusion could possibly occur, and in the end 
each lot was again carefully analyzed as a double 
check on the operations. 

One ingot of each grade of open-hearth was rolled 
into 16-gauge and the other into 27-gauge sheets, 30 
by 96 inches; while in the case of the Bessemer steel, 
one-half of each ingot was rolled into 16- and the 
other half into 27-gauge. All grades were subjected 
to exactly the same treatment, being rolled by the 
same crews, and annealed in the same furnace at the 
same time, and the finish was such as to conform 
with that of the competitive sheets used in this test. 
From 24 to 36 sheets of each of the 9 grades, both 
gauges, making 18 lots in all, were then sheared to 
24 by 96 inches, thus obtaining a strip 6 inches wide 



142 COEROSIOIT OF IRON" 

from each sheet. These strips were sheared into 2 hy 
4 inch test pieces, stenciled with distinguishing 
marks, and were used for corrosion tests which will 
be described later. The 24 by 96 inch sheets were 
corrugated in the usual way, and eight to twelve 
sheets of each grade shipped to each of three testing 
stations. One of these is located in the Pennsyl- 
vania coke regions, where the air contains notable 
amounts of sulphurous and sulphuric acids and other 
fumes from the coke ovens. In this district iron and 
steel, unless protected, corrode very fast. Another 
station is located on the sea coast, where the air car- 
ries sodium chloride. The third is in a rural com- 
munity, where the air is quite pure and free from 
added corrosive agents. 

A sloping wooden framework was erected 
at each place and the sheets fastened thereon, 
with the precautions to see that the drip 
from one sheet did not fall on the next one 
below. They were thus held several feet 
from the ground, so that there was a free cir- 
culation of air on all sides, and exposed to 
the weather without a protective coating of 
any sort except the thin film of oxide nor- 
mally present on the surface of black sheets. 

There were likewise exposed at the same 
time and under the same conditions sheets of 
both 16- and 27-gauge, purchased on the open 
market and showing the following average 
analysis : 



ELEMENTS INFLUENCING CORROSION 143 

Carbon Manganese Sulphur Phosphorus Copper 



.02 .03 .034 .003 .06-.07 

All the sheets were placed in position dur- 
ing November, 1911, and were frequently in- 
spected ; by means of the small test pieces cut 
from the large sheets and exposed at the same 
time, it was possible to determine the loss of 
weight of each grade. 

First, considering briefly the results of the 
examinations of the large sheets after vary- 
ing periods of exposure, at the end of seven 
months in the coke regions the Bessemer steel 
sample without copper had failed entirely, 
while those from the same heat but with cop- 
per additions were in very good condition. 
The same thing was true of the basic open- 
hearth steels, the copper-bearing samples 
showing up well, while those without it had 
failed. One from a re-phosphorized heat, but 
containing no copper, was in somewhat better 
condition than the corresponding panels from 
the other heats, although it was showing the 
effects of corrosion, while the copper-bearing 
panels were in good shape. 

It was noted that the color of the oxide on 
the non-copper steels was bright red and 
loosely adherent. The copper steels were 



144 



CORROSION OF IRON" 






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ELEMENTS INFLUENCING CORROSION 



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146 



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ELEMENTS INFLUENCING CORROSION 147 

dark brown and the oxide adhered tena- 
ciously. 

An inspection several months later dis- 
closed that the steels without copper had 
failed completely and fallen to the ground, 
while the copper steels were still in fair 
shape. The low-carbon and manganese ma- 
terial, American ingot iron, was in rather 
bad shape and after about eighteen months 
of exposure had disappeared entirely al- 
though all of the copper-bearing sheets were 
still in place. 

While the samples at the sea-shore exhib- 
ited relatively less rapid corrosion than those 
in the coke regions, the same differences in 
the behavior of the copper and non-copper 
steels were shown here as in the other case. 
In something less than two years all the nor- 
mal steel samples had failed completely, 
which was also true of the American ingot 
iron, with the exception of two sheets, while 
the copper-bearing steel panels were still in- 
tact. 

At the test station located in the country 
district, too, the copper steels showed up to 
approximately the same advantage, over the 
normal materials, as at the other stations. 
Space does not, however, permit of a detailed 



148 CORROSION OF IRON 

discussion of the results of the inspections 
and in any case a clearer idea of the showings 
of the various materials under test may per- 
haps be obtained from a consideration of the 
results obtained on the small test pieces. It 
may be noted that each result in the preced- 
ing table is the average of six pieces ; as men- 
tioned previously these were 2 by 4 inches in 
size. 

These results need no comment or explana- 
tion, but it is apparent that some progress in 
the right direction has been made. Whether 
it will be possible sometime so to alter the 
characteristics of iron, by the addition of 
some other metal or combination of metals, 
that it will not rust at all or only with diffi- 
culty, and at the same time not change its 
mechanical properties objectionably, seems to 
be doubtful, although further advances along 
this line will probably be made as time goes 
on. 

Just now, the best available material of 
this sort is copper-bearing steel, which is 
finding a wide and rapidly growing use ; any- 
one can make it and the present output is 
very large. 



Chapter YII 

THE COREOSION OF WEOUGHT lEOIST AND 
STEEL PIPE 

T T has been shown in the preceding chapter 
-*• that iron and steel may be made less sus- 
ceptible to corrosion by the addition of cer- 
tain elements, but it is a matter of common 
experience that even without these additions 
some parts are more resistant to such attack 
than others, and brief mention has already 
been made of the fact that the iron manu- 
factured today is not quite the same, in some 
respects, as that produced formerly. The 
substitution of modern methods and machine 
processes for the old, hand way of refining 
these materials has resulted in a different 
product, both chemically and physically, and 
coincident with these changes in method and 
product there has been a tremendous increase 
in the use of iron and steel, and perhaps an 
equally great alteration of the conditions of 
service ; so it is not strange that a good deal 
of difference of opinion, doubtless honest 

149 



150 CORROSION" OF IRON" 

enougli at bottom, has arisen regarding the 
relative merits of the old and new varieties, 
especially their resistance to corrosion. The 
controversy over wr ought-iron and steel pipe, 
for instance, has been very animated. 

Perhaps one reason for this is that nearly 
everyone uses pipe, or comes in contact with 
it in some way ; then the conditions to which 
a steam or water pipe are exposed are rather 
unusual and just the opposite of what gen- 
erally obtains. On the whole, ordinary iron 
work is subjected to a large excess of air 
containing varying amounts of moisture and 
gases; only at long intervals, as from rain 
and snow, does the surface become wet. The 
inside of a pipe line, on the other hand, is in 
contact with a large excess of water, which 
may contain much or little oxygen or air. 
In a line where the water is frequently 
drained out of the pipes, or where it holds 
a large amount of air, it will be seen that 
the conditions for rapid and severe corrosion 
are present and it is not surprising that a 
line, or section of it, may go to pieces in a 
comparatively short time. "When such a case 
happens it is very easy, and perhaps natural, 
to ascribe it to inferior materials or some- 
thing of that sort, and unless trouble is taken 



IRON AND STEEL PIPE 151 

to ascertain all the facts in the case very er- 
roneous notions may, and often do, become 
current. Such a one is the belief held by 
some that steel pipe is considerably inferior, 
in point of corrodibility, to that made from 
wrought iron. 

There is perhaps no doubt that the steel 
produced ten or fifteen years ago was not 
the equal in all respects of that turned out to- 
day, but by reason of the improvements 
which are being introduced from time to time 
the quality has risen until it may safely be 
said that there is no ground for comparing 
it unfavorably with wrought iron, at least 
without a fair trial. 

It may be remarked that this subject is by 
no means new, and there is available a good 
deal of literature concerning it, but it is con- 
sidered to be worth while to review some of 
this briefly and, without in the least desiring 
controversy with any whose opinions or in- 
terests lead them to disagree with the con- 
clusions stated therein, to present the results 
of various investigations in the hope that 
this may help to clear up any false impres- 
sions. Much money is lost every year 
through ignorance and prejudice, and at least 
some of this occurs in buying ferrous metals, 



152 COEEOSIOX OF lEOJf 

since certain varieties of these are claimed to 
be superior to others and a higher price is 
charged for them. If the facts show that this 
extra premium is warranted, well and good; 
if not, it is a sheer waste of money. 

Before considering the data bearing on 
this point, however, it may be well to take 
up briefly the processes employed in the 
manufacture of these two materials so that 
a more definite idea of what each is may be 
obtained. 

The way in which iron ore is reduced in 
the blast furnace, with the production of pig 
iron containing several per cent of other met- 
als and metalloids, has been previously de- 
scribed. As noted, this does not permit of 
rolling or similar treatment; therefore, it 
must be refined before it is suitable for use 
other than in making castings. Iron may be 
refined in several ways, but for our purpose 
only the puddling furnace and Bessemer con- 
verter need be considered, as the terms 
wrought iron and steel, as used in the manu- 
facture of pipe, refer to the product of these. 

In puddling, the iron is melted on a bed of 
iron ore which gives up part of its oxygen 
to oxidize the carbon, silicon and manganese 
and, to a less extent, the sulphur and phos- 



IRON AND STEEL PIPE 153 

phorus. The greater the purity of iron, the 
higher the temperature at which it melts; 
therefore, the metal in the furnace becomes 
pasty after a time, as the impurities are 
burned out, and is worked into a ball and re- 
moved. 

This is rolled into bars of convenient size 
which are cut and piled up, then reheated to 
a welding temperature and rolled again into 
a bar of the desired dimensions. 

Wrought iron produced in this manner is 
very pure, the greatest and most objection- 
able part of the impurities being the slag 
which is scattered all through the metal and 
gives it the apparently fibrous structure so 
characteristic of it after rolling. These fine 
slag lines are easily detected and form a 
ready means of distinguishing between 
wrought iron and that produced by other 
methods ; it is only necessary to smooth down 
a small spot with a file when they will show 
up, best under a magnifying glass. 

It can be seen that the puddling process is 
rather slow and expensive, since only a lim- 
ited amount of metal, a few hundred pounds 
at most, can be treated at a time, while some- 
thing like an hour and a half is required for 
a charge and considerable skill and care are 



154 CORROSION OF IRON 

needed in order to obtain a good product. 
The presence of 2 per cent or so of slag dis- 
tributed irregularly throughout the mass can 
hardly tend to increase the homogeneity, and 
it is generally accepted that lack of this is of 
more importance, from the standpoint of cor- 
rosion, than the presence of a reasonable 
amount of impurities thoroughly and uni- 
formly mixed in. Further, it is very hard to 
weld perfectly sheets or bars several inches 
in width, so laminations are met with and 
may give serious trouble. 

In the Bessemer process the molten iron 
from the blast furnace is run into huge mix- 
ers, then into big converters, shaped some- 
thing like an egg, wherein cold air is blown 
through it, entering by means of tubes in the 
bottom. The oxygen rapidly burns out the 
impurities, thereby generating a large 
amount of heat, so no difficulty is experienced 
in keeping the metal melted. The length of 
time the air is blown through is determined 
by the color of the flame issuing from the 
mouth of the converter. As might be ex- 
pected, more or less of the iron is oxidized 
at the same time ; but the addition of a small 
amount of manganese, for example, will take 
care of these oxides, so the requisite quan- 



lEON AND STEEL PIPE 155 

tity of tMs, calculated from an analysis of 
the iron, is added, together with such other 
elements as may be needed. 

It requires only ten or fifteen minutes to 
convert as many tons of iron into steel, so in 
point of speed and general economy the Bes- 
semer process is superior to the other; it is 
especially adapted to making low-carbon 
steel and, as a matter of fact, gives a prod- 
uct which is more nearly pure, considered on 
the score of total impurities, than that from 
the puddling furnace. As a general proposi- 
tion, the silicon, carbon, sulphur, manganese, 
and phosphorus may be lower in the wrought 
iron than in steel, but the former will contain 
about two per cent of oxides while in steel 
these will not run over one- or two-tenths of 
one per cent. From the nature of the proc- 
ess whatever impurities are present are well 
distributed, and as the metal is rolled directly 
into skelp, without cutting and re-welding as 
in the case of wrought iron, the danger from 
laminations is greatly reduced. 

Up to about twenty-eight years ago hand- 
puddled iron was used entirely for making 
pipe ; for various reasons steel had not been 
successfully welded, and as the quantities 
needed were comparatively small there was 



156 COEEOSION OF lEON 

no trouble in supplying the demand. With an 
increasing use of pipe, however, the discov- 
ery that steel could be welded when treated 
properly, and the discovery of cheaper and 
better processes of making this material, 
caused it to be employed more and more and 
two classes of pipe appeared on the market, 
iron and steel. With many manufacturers in 
the field different grades of these appeared, 
and now there are a number of such ; in fact, 
there is as much difference between some of 
the wrought irons as there is between 
wrought iron and the steels, and there is a 
corresponding disparity between the latter. 
All of this has tended to increase the confu- 
sion; and inasmuch as any new material is 
liable to be looked upon with suspicion, it is 
perhaps only natural that there should have 
arisen much lack of agreement regarding the 
length of life which iron pipe, as contrasted 
with that made from steel, might be expected 
to give. 

As a consequence a good deal of experi- 
mental work has been done and several in- 
vestigations of the conditions found in actual 
service carried out to find the answer to some 
of these questions. 

An experiment conducted somewhat over 



IRON AND STEEL PIPE 



157 



twelve years ago by the Bureau of Steam En- 
gineering of the Navy Department on lap- 
welded Bessemer steel, lap-welded iron, seam- 
less cold-drawn steel and seamless hot-drawn 
steel boiler tubes, may be of interest. 

Care was taken to secure good, representa- 
tive samples of these materials and analysis 
showed them to be of the following average 
composition : 





ANALYSIS 




Cold- 
drawn 
seamless 


Hot- 
drawn 
seamless 


Bessemer 
steel 


Iron 


Silicon 


.005 

.043 

.015 

.50 

.16 

.23 


.005 

.039 

.016 

.49 

.14 

.30 


.008 

.081 

.110 

.31 

.063 

.28 


.024 


Sulphur 


.014 


Phosphorus 


.038 


Manganese 


Trace 


Carbon 


Trace 


Oxide 


1.03 







Without going too much into detail, the 
test consisted in weighing the parts, then ini- 
mersing them in distilled water, through 
which air was bubbled, for a total of 64 weeks, 
divided into four periods of sixteen weeks 
each. At the end of each period the sam- 
ples were removed, thoroughly cleaned and 
weighed. The results may be averaged thus : 



158 



COEKOSION OF IRON 



AVERAGE LOSS IN GRAMS PER SQTTABE INCH 





After 

16 
Wks. 


After 

32 
Wks. 


After 

48 
Wka. 


After 

64 
Wks. 


Total 
Av. 

Loss 


Compar- 
ing Iron 
as 100 


Hot-d'n 0. H. Steel 

Bessemer Steel 


. 30.3-i 
.3147 


. 5333 
.4950 
.5564 
.5896 


.3510 
.4043 
.4502 
.3966 


.5070 
.4945 
.5017 
.4893 


1.6947 
1.7085 
1.8315 
1.8081 


93.7 
94.5 


Cold-d'n 0. H. Steel 

Charcoal Iron 


.3232 
.3326 


101.3 
100. 









It will be seen by comparing the figures of 
loss given above that there was no great 
difference in the behavior of these four 
classes of materials, as far as loss in weight 
is concerned; the charcoal-iron samples, 
which showed the greatest initial loss, ending 
up with a decrease about 6.3 per cent larger 
than that of the open-hearth hot-drawn tubes, 
which suffered the least. 

From data collected by the American So- 
ciety for Testing Materials, 1908, and experi- 
ments reported in the Iron Age, the table on 
page 159 has been drawn up summarizing the 
results of a number of other laboratory tests, 
largely on wrought iron and steel skelp. The 
specimens were exposed to the weather or im- 
mersed in different kinds of water and the 
loss in weight determined at the end of the 
experiment. 

These results show that even the steel man- 
ufactured fifteen years ago was slightly bet- 



IRON AND STEEL PIPE 



159 



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160 cokeosio:n' of iron" 

ter than the wrought and charcoal irons of 
the same period. 

But while laboratory tests are useful, in 
giving indications of what may be expected 
in practice, the results obtained in actual 
service are naturally more reliable, so about 
the time the last of the tests were being car- 
ried out, a new line of investigation in the 
shape of trial installations was started. 

Prof. Thomson put in a hot-water line, 
using wrought-iron and steel pipe alter- 
nately, so that they would all be subjected to 
the same conditions. After a year of service 
he concluded that steel might be expected to 
stand up about 7.5 per cent longer, under 
such conditions, than wrought iron. 

A short time later a test carried out under 
similar circumstances under the auspices of 
the American Society of Heating and Ven- 
tilating Engineers, using steel and iron pipes 
supplied by; several makers, checked up 
closely enough with the previous one and the 
report of the trial concludes : 

We beheve (this test) demonstrates that modem 
steel pipe of good quality is at least as durable as 
modern strictly wrouglit-iron pipe of good quality 
and is very much superior to a poor quality of 
wrought iron in this class of work. 



IRON AND STEEL PIPE 161 

Various other tests carried out by different 
observers under a variety of conditions have 
led to the same general conclusion. Thus, 
several experiments conducted by the Pitts- 
burg Coal Co. and the H. C. Frick Coke Co., 
to mention only two, wherein the samples 
were immersed in running mine water, which 
generally contains considerable quantities of 
acid, show that steel corrodes at no greater 
rate than wrought iron. Again, the work of 
Howe and St ought on gave results in broad 
agreement with this. Commenting on the 
tests of these two experimenters and the evi- 
dence which they have collected, Alfred Sang 
says in his book: 

Of ten different tests made by different observers 
in different places, seven resulted decisively in favor 
of steel; in the other three cases the results were 
very slightly in favor of the iron, but in only one 
of the latter was the material of modern manufac- 
ture. 

The tests which resulted in favor of steel were as 
follows, all except the two first being carried to de- 
struction : Seven months in hot, aerated salt water ; 
sixteen months buried in dampened ashes; exposed 
to sulphuric acid coal-lime water; in railroad inter- 
locking and signal service; in locomotive boiler 
service. 

It was also found that steel tubes made in 1906 
pitted much less than those of 1897 from the same 
makers, indicating the superiority of modern steel 



163 CORROSION OF IRON 

over that of some years back in this particular re- 
spect. 

Not all of the work on this subject has been 
confined, however, to either laboratory or 
short installation tests, and at least two 
rather extended investigations have been un- 
dertaken on pipe lines as they are found in 
service. The first was conducted by Prof. 
Woolson on some of the public bath houses of 
New York City, where much trouble was be- 
ing encountered from the rapid corrosion and 
consequent leaking of the hot-water pipes. 

A thorough examination of the situation 
showed that conditions in these bath houses 
were not materially different from those 
found in similar installations in other places. 
Croton water was used, being heated in vari- 
ous standard types of heaters to a tempera- 
ture of 160 degrees to 200 degrees F. The 
quantity of water passing could not be de- 
termined, but it was considerable, as many 
thousands of baths were, and are, given in 
some of these places during the hot weather. 
It developed, further, that all the pipe which 
had been in use four years or over had be- 
gun to give trouble, and in a few places pipe 
only two or three years old had perforated. 

In all, eleven houses were inspected, but for 



IRON AND STEEL PIPE 



163 



various reasons samples could be collected 
from only eight ; from these eighty-nine spe- 
cimens, perforated and worthless, were ob- 
tained for examination. They varied in size 
from % inch to 4 inches in diameter, the ma- 
jority being II/2 to 2 inches. 

Small pieces were sawed from each of the 
specimens, crushed, and the fracture care- 
fully examined by two experts independently 
to determine whether the material was 
wrought iron or steel ; chemical analyses were 
then made of all samples which were at all 
doubtful in classification. The results of 
these tests were further checked to avoid all 
possibility of error. 

ANALYSES OF PIPE FROM NEW YORK CITY BATH HOUSES 



Manganese, Per Cent 



Carbon, 


Oxide, 


Per Cent 


Per Cent 


.09 


.20 


.08 


.26 


.08 


.21 


.08 


.30 


.08 


.38 


Trace 


2.85 


Trace 


2.40 


Trace 


2.20 


Trace 


2.50 


Trace 


1.75 



Classified as 



.48.. 
.32.. 
.42.. 
.38.. 
.50.. 

Trace 
Trace 
Trace 
Trace 
Trace 



Steel 
Steel 
Steel 
Steel 
Steel 

Wrought iron 
Wrought iron 
Wrought iron 
Wrought iron 
Wrought iron 



164: 



CORROSION OF IRON 



As a matter of interest, in showing the dif- 
ference between the two kinds of metal, the 
table on page 163 presents typical results of 
the chemical analysis. 

From all this work it was decided that sev- 
enteen out of the eighty-nine samples were 
wrought iron; these had come from six of 
the eight houses yielding samples. Of the 
other two, no samples were examined from 
one and those collected from the last were 
proved to be steel. The above data may be 
arranged in tabular form as follows : 

BATH HOUSES AND SAMPLES OBTAINED FROM EACH 



Material Supplied 



Probably steel , 
Probably steel, 
Not certain . . . 

Steel 

Steel 

Steel 



Bath House 



nth St. 
76th St. 
23d St. 
41st St. 
Rivington St. 
109th St. 



Classification 



2 
2 

2 
5 
2 
4 

17 



Iron, 6 Steel 
Iron, 5 Steel 
Iron, 4 Steel 
Iron, 7 Steel 
Iron, 23 Steel 
Iron, 11 Steel 

56 



Total of mixed samples ; . ;....;... 73 

Steel, Allen Street 16 Steel 

Total of all samples 89 

The contract specifications for all except 
two of these houses called for wrought-iron 
pipe, galvanized, but plumbers and dealers 



IRON AND STEEL PIPE 165 

often do not make any close distinction be- 
tween iron and steel pipe, so the latter may 
be supplied alone or mixed with the other. A 
few years ago many mills were making both 
kinds of pipe and it would be very easy to 
mix them in shipment. 

However that may be, it will be seen that 
of the mixed samples 23 per cent were 
wrought-iron and, bearing in mind that the 
two kinds of pipe were used indiscriminately, 
therefore all exposed to the same conditions, 
that all samples were tested to destruction 
and that probably not more than 10 per cent 
of the welded pipe on the market is wrought 
iron, it appears that steel showed up to very 
fair advantage. 

After his investigation Prof. Woolson de- 
cided : 

In my judgment, from the evidence collected, there 
was absolutely no difference in the corrosion of the 
two classes of pipe. They appeared to be equally sus- 
ceptible to the attack. 

A similar study was made by Dr. Walker 
in an attempt to determine how these ma- 
terials stood up in service when both were 
used in the same system, being separated 



166 CORROSION OF IRON 

from each other only by a coupling. Under 
these circumstances the wrought-iron and 
steel pipe would be subjected to as nearly 
identical conditions as could be obtained, so 
a fair basis of comparison should be af- 
forded. 

Briefly, the method pursued was to exam- 
ine a large number of steam and hot-water 
systems operating in different places and un- 
der different conditions and to collect samples 
of wrought iron and steel pipe. It chanced 
that the majority of the samples collected 
came from hot and cold-water feed systems, 
but enough were obtained from live and ex- 
haust-steam lines and other installations to 
make the results of general interest. 

The specimens were split lengthwise and 
carefully cleaned of rust and accumulated 
scale by the use of ammonium-citrate solu- 
tion, which does not attack iron, after which 
the extent of corrosion was calculated by 
measuring with a micrometer, the ten deepest 
pits occurring within a length of about 12 
inches. 

In all, it was possible to obtain sixty-four 
comparisons between iron and steel and the 
results may be summed up as shown in the 
table at the top of page 167. 



"f 



IKON AND STEEL PIPE 167 

COMPARATIVE CORROSION OF WROUGHT-IRON AND 

STEEL PIPE 

Instances where iron corroded more than steel. . 20 

Instances where steel corroded more than iron. . 18 

Instances where iron and steel corroded equally . 9 

Instances where corrosion was negligible 17 

In the light of the facts brought out by this 
work, Dr. Walker felt justified in concluding 
that on the average there is no difference in 
the corrosion of iron and steel pipe. 

Immersion in sulphuric acid of a certain 
strength has been used as a means of deter- 
mining the comparative resistance to corro- 
sion of different samples of iron and steel, 
and as an interesting sidelight to the above 
work. Dr. Walker conducted some experi- 
ments to find out whether this test has any- 
real value. As he reports it: 

In order to show what relation may exist between 
the so-called acid corrosion test and the real corro- 
sion as found in service, eleven pairs of iron and 
steel were selected and subjected to 20 per cent sul- 
phuric acid for four hours at room temperature. 
Four pairs were selected in which the steel was de- 
cidedly better than the iron in service, four in which 
the iron had shown decidedly better than the steel, 
and three in which there was no difference between 
the two metals. 

In six instances the relative corrosion as shown 
by the sulphuric acid test corresponded with the cor- 



168 CORROSION OF IRON" 

rosion as found in service. In five instances corro- 
sion as shown by the acid test was exactly contrary 
to that found in service. Although the greatest care 
was taken to have the specimen of the same size, 
cleaned in the same way, and in the same physical 
condition, the results show that no reliance can be 
placed in this accelerated acid test, but that it may 
be entirely erroneous and very misleading. Not only 
did the acid test not agree with the service test when 
steel was compared with iron, but the steels failed 
to agree among themselves, and the irons showed no 
agreement when considered by themselves. 

It may be added that these conclusions 
have been reached by other investigators, 
working independently. After all, it appears 
that, no matter whether one is testing pipe 
or paint or almost anything else, an actual 
service test is the only really reliable crite- 
rion of merit, or the lack of it. 

Viewing impartially all of the data pre- 
sented so far, there seems to be little to 
choose between wrought-iron and steel pipe, 
on the whole, as regards their resistance to 
corrosive influences, but one point may be 
mentioned with reference to the manner in 
which these materials corrode. With steel 
the rusting takes place more or less uniformly 
over the surface, while wrought iron shows 
a decided inclination to form deep pits. That 
this is a dangerous tendency can hardly be 



lEON" AND STEEL PIPE 169 

doubted : to paraphrase an old saying, a pipe 
wall is no stronger than its thinnest spot; 
therefore, to the extent in which wrought iron 
exhibits this defect in greater measure than 
steel, it may be considered correspondingly 
inferior to the latter. 

In the course of the investigation men- 
tioned above, Dr. Walker found that in the 
most corroded samples (those being practi- 
cally worthless) the average depth of pitting 
was .130 of an inch for the iron and .118 for 
the steel. Likewise, it was found that of 
twenty-six samples of iron pipe removed 
from hot-water boiler-feed lines in the power 
plants of the H. C. Frick Coke Co. the aver- 
age of the deepest pit in each was .112 of an 
inch, while the average of twenty-six steel 
samples taken from the same lines was .108. 
Of course, these differences are not at all 
startling, but they would seem to indicate a 
weak point in wrought pipe. 



THE EOT) 



INDEX 

PAOB 

Acid test, as measure of corrodibility 167 

Alkaline solutions, effect on corrosion 26 

Alloys, as protective coatings 73 

" hydrogen-iron 40 

Aluminum, effect on corrosion 135 

" silicate, China clay 95 

American vermilion 96 

Ancient irons 7 

Angus Smith method of protecting pipe 120 

An-ions 14 

Annealing 5, 8 

Arsenic, effect on corrosion 136 

Asbestine, magnesium silicate 95 

Asbestos Protected Metal 117 

Asphaltum, as protective coating 117 

Assembling iron parts of different polarity. ... 38 

Atlantic City, paint tests at 109, 112, 113 

Baking japans 118 

Barium chromate 88 

Barytes, barium sulphate 97 

Basic carbonate, white lead 91 

Benzol 82 

Bessemer process 154 

Bone black 95 

Bontempi process 50 

Boiler tubes, tests on materials for 157 

Bower-Barff process 48 

171 



172 INDEX 

FAGB 

Bradley process 60 

Buffington process 51 

Calcium carbonate, wMting 92 

" sulphate, gypsum 92 

Carbon, effect on corrosion 133 

Carbon black 98 

Carbonic acid theory of corrosion 25 

Cat-ion 15 

China clay, aluminum silicate 95 

China wood oil, tung oil 80 

Chrome green 94 

" " blue tone 89 

Chromic acid, as rust inhibitor 25, 42 

Cleaning of parts before painting 125 

Cobalt, effect on corrosion 136 

Colloids 21 

Combinations of pigments 116 

Copper, effect on corrosion 139 

" electro-deposition of 69 

Copper-clad steel 69 

Copper sulphate solution, action on iron 30 

*^ " *^ standard for Preece 

test 57 

Corrosion due to attack of hydrogen ions 31 

" effect of aluminum on 135 

" ^' " arsenic on 136 

^' '^ ^^ carbon on 133 

'' " " cobalt on 136 

" " " copper on 139 

^* " '^ iron oxide on 38 

ic (c a manganese on 132 

" " " nickel on 137 

*^ " " phosphorus on 133 

'' " " pickling on 39 

" ^^ " platinum wire on 37 



INDEX 173 

FAOB 

Corrosion, effect of segregation of impurities on 5 

" " " selenium on 138 

'' " " silicon on 131 

" " " silver on 138 

'' " " sulphur on 132 

'^ ^^ ^^ stresses and strains on ... . 5 

'' " " tin on 138 

'' " " tungsten on 138 

" " '' vibration on 41 

" *^ " wounds on 44 

" " " zinc on 37 

^^ of iron parts in the ground 41 

" " " " '' running water 41 

^^ relative, of wrought iron and steel. . . 161 

'^ resistance to, of pure iron 128 

" theories of, carbonic acid 25 

" " " electrolytic 29 

*' " '^ hydrogen peroxide 23 

*^ varying tendency of irons and steels 

to 4 

Coslettizing process 52 

Crystalloids 21 

Dewees-Wood process 50 

Dissociation, theory of 15 

Driers, oil and japan 78 

" use of 79 

Electro-deposition of copper 69 

" " nickel 69 

" " " zinc 60 

Electrolyte 17 

Electrolytic iron 134 

" theory of corrosion 29 

Electro-plating 18 

Enamels 119 



174 rsDzx 

PAGE 

Examination of pipe lines in Xew York City 

bath-houses 162 

Examination of pipe lines in various installa- 
tions 165 

EerToxyl reagent 33 

Eish oil;, menhaden oil 80 

Galvanizing, electro 60 

^' hot dip method 55 

Gresner process 49 

Graphite 97 

Gypstim, calcium snlphate 92 

Havre de Grace bridge, paint tests on 105 

Heat bluing 52 

Hydrogen, alloy vith iron 40 

" as a' metal 30 

" peroxide theory of corrosion 23 

Hydrolysis 16 

Hydroxyl 15 

Indeterminate pigments 86 

Indian red 96 

Indicators 33 

Inhibitive pigments 86 

Ions, formation of 14 

Iron, alloy ^th hydrogen 40 

'-' electrolytic ". 134 

" impurities in 9, 130 

" ore, reduction in blast furnace 129 

" oxide, effect on corrosion 38 

" piue, resistance to corrosion 128, 135 

" refining of 152 

" " ^' Bessemer process 154 

" '' ptiddling " 152 



INDEX 175 

PAOB 

Iron, sulphide and phosphide films as protec- 
tive coatings 51 

" wrought 153 

Japan drier 78 

Japans, baking 118 

Keystone filler 97 

Lacquers 123 

Lampblack 98 

Lead, basic carbonate white 91 

" protective coatings of 70 

'' red 95 

" sublimed blue 93 

'' " white 96 

Lemon chrome yellow 93 

Linseed oil i. . 76 

" " boiling of 77 

'^ " film, as depolarizer 44 

Litharge 90 

Lithopone 89 

Magnesium silicate, asbestine 95 

Manganese, effect on corrosion 132 

" " " properties of iron 10 

Medium chrome yellow 94 

Menhaden oil, fish oil 80 

Mineral black 97 

Molten zinc process 65 

National Coating, for pipe. 122 

Newburyport bridge 6 

Nickel, effect on corrosion 137 

" electro-deposition of 69 



170 IITDEX 

PAGB 

Ochre 98 

Oil drier 78 

Oils, China wood, tiing 80 

" linseed 76 

" menhaden, fish 80 

'^ soya bean 80 

Old iron parts 6 

Orange chrome yellow 93 

" mineral 92 

Osmotic pressure 13, 14 

Oxidation 20 

Oxides in steel 155 

" " wrought iron 155 

Paint films, excluding 101 

" porosity of 101 

^^ varnish in 104 

manufacture of 75 

tests, at Atlantic City 109, 112, 113 

" on Havre de Grace bridge 105 

Passive condition 42 

Phenolphthalein, indicator 28 

Phosphorus, effect on corrosion 133 

Pickling, " " " 39 

Pigments, combinations of 116 

" indeterminate 86 

^^ inhibitive 86 

" stimulative 86 

*' properties of 94 

Pipe lines, examination of 162, 165 

" " service tests of 160 

Potassium ferricyanide, in ferroxyl reagent. . . 33 

Preece test 57 

Princes metallic brown 92 

Protective coatings, classes of 46 



INDEX 177 

FAGB 

Prussian blue 89 

Puddling process 152 

-tieCl 16H.(1 • • • • • •■• ••••• ••• oo 

Eeduction 20 

Eelative corrosion of wrought iron and steel. . 161 

Sabin process, for pipe 121 

Schoop process 71 

Segregation of impurities, effect on corrosion . . 5 

Selenium, effect on corrosion 138 

Sherardized iron, structure of 63 

Sherardizing , 63 

Silicon, effect on corrosion 131 

Silver, " " '' 138 

Solution, phenomenon of 11 

" pressure 12 

" " effect on, of strains and 

segregation of impurities. 32 

Soya bean oil 80 

Steel 155 

Stimulative pigments 86 

Stresses and strains, effect on corrosion 5 

Sublimed blue lead 93 

" white " 96 

Sulphur, effect on corrosion 132 

Theory of dissociation 15 

" " corrosion, carbonic acid 25 

" " " electrolytic 29 

" " " hydrogen peroxide 23 

Tin, effect on corrosion 138 

Tung oil, China wood oil 80 

Tungsten, effect on corrosion 138 

Turpentine 81 



178 INDEX 

PAGE 

Ultramarine blue 88 



Valence . . . .:r. 19 

Varnish film as depolarizer 43 

" in paint, effect of 104 

Venetian red 94 

Vibration, effect on corrosion 41 

Walker's test for porosity of zinc coatings. ... 67 

Wells process 49 

Willow charcoal 90 

White lead, basic carbonate 91 

" " sublimed 96 

Whiting, calcium carbonate 92 

Wounds, effect on corrosion 44 

Wrought iron 153 

" " and steel, relative corrosion of . . 161 

« '' " " skelp, tests on 159 

pipe, service tests on. 160 



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Zinc and barium chromate 87 

" chromate 87 

coatings, weight of 57, 62, 67 

effect of on corrosion ■.. . . •- 37 

electro-deposition of •..-, 60 

lead white 88 

oxide .1. 87 



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